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  1. 25 Jun, 2020 18 commits
  3. 24 Jun, 2020 7 commits
  5. 23 Jun, 2020 15 commits
    • Alexei Starovoitov's avatar
      Merge up to bpf_probe_read_kernel_str() fix into bpf-next · 4e608675
      Alexei Starovoitov authored
      4e608675
    • Tobias Klauser's avatar
      tools, bpftool: Correctly evaluate $(BUILD_BPF_SKELS) in Makefile · 9d9d8cc2
      Tobias Klauser authored 
      Currently, if the clang-bpf-co-re feature is not available, the build
fails with e.g.

  CC       prog.o
prog.c:1462:10: fatal error: profiler.skel.h: No such file or directory
 1462 | #include "profiler.skel.h"
      |          ^~~~~~~~~~~~~~~~~

This is due to the fact that the BPFTOOL_WITHOUT_SKELETONS macro is not
defined, despite BUILD_BPF_SKELS not being set. Fix this by correctly
evaluating $(BUILD_BPF_SKELS) when deciding on whether to add
-DBPFTOOL_WITHOUT_SKELETONS to CFLAGS.

Fixes: 05aca6da ("tools/bpftool: Generalize BPF skeleton support and generate vmlinux.h")
Signed-off-by: default avatarTobias Klauser <tklauser@distanz.ch>
Signed-off-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
Reviewed-by: default avatarQuentin Monnet <quentin@isovalent.com>
Acked-by: default avatarAndrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200623103710.10370-1-tklauser@distanz.ch
      9d9d8cc2
    • John Fastabend's avatar
      selftests/bpf: Add variable-length data concat pattern less than test · 2fde1747
      John Fastabend authored 
      Extend original variable-length tests with a case to catch a common
existing pattern of testing for < 0 for errors. Note because
verifier also tracks upper bounds and we know it can not be greater
than MAX_LEN here we can skip upper bound check.

In ALU64 enabled compilation converting from long->int return types
in probe helpers results in extra instruction pattern, <<= 32, s >>= 32.
The trade-off is the non-ALU64 case works. If you really care about
every extra insn (XDP case?) then you probably should be using original
int type.

In addition adding a sext insn to bpf might help the verifier in the
general case to avoid these types of tricks.
Signed-off-by: default avatarJohn Fastabend <john.fastabend@gmail.com>
Signed-off-by: default avatarAndrii Nakryiko <andriin@fb.com>
Signed-off-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200623032224.4020118-3-andriin@fb.com
      2fde1747
    • Andrii Nakryiko's avatar
      selftests/bpf: Add variable-length data concatenation pattern test · 5e85c6bb
      Andrii Nakryiko authored 
      Add selftest that validates variable-length data reading and concatentation
with one big shared data array. This is a common pattern in production use for
monitoring and tracing applications, that potentially can read a lot of data,
but overall read much less. Such pattern allows to determine precisely what
amount of data needs to be sent over perfbuf/ringbuf and maximize efficiency.
Signed-off-by: default avatarAndrii Nakryiko <andriin@fb.com>
Signed-off-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200623032224.4020118-2-andriin@fb.com
      5e85c6bb
    • Andrii Nakryiko's avatar
      bpf: Switch most helper return values from 32-bit int to 64-bit long · bdb7b79b
      Andrii Nakryiko authored 
      Switch most of BPF helper definitions from returning int to long. These
definitions are coming from comments in BPF UAPI header and are used to
generate bpf_helper_defs.h (under libbpf) to be later included and used from
BPF programs.

In actual in-kernel implementation, all the helpers are defined as returning
u64, but due to some historical reasons, most of them are actually defined as
returning int in UAPI (usually, to return 0 on success, and negative value on
error).

This actually causes Clang to quite often generate sub-optimal code, because
compiler believes that return value is 32-bit, and in a lot of cases has to be
up-converted (usually with a pair of 32-bit bit shifts) to 64-bit values,
before they can be used further in BPF code.

Besides just "polluting" the code, these 32-bit shifts quite often cause
problems for cases in which return value matters. This is especially the case
for the family of bpf_probe_read_str() functions. There are few other similar
helpers (e.g., bpf_read_branch_records()), in which return value is used by
BPF program logic to record variable-length data and process it. For such
cases, BPF program logic carefully manages offsets within some array or map to
read variable-length data. For such uses, it's crucial for BPF verifier to
track possible range of register values to prove that all the accesses happen
within given memory bounds. Those extraneous zero-extending bit shifts,
inserted by Clang (and quite often interleaved with other code, which makes
the issues even more challenging and sometimes requires employing extra
per-variable compiler barriers), throws off verifier logic and makes it mark
registers as having unknown variable offset. We'll study this pattern a bit
later below.

Another common pattern is to check return of BPF helper for non-zero state to
detect error conditions and attempt alternative actions in such case. Even in
this simple and straightforward case, this 32-bit vs BPF's native 64-bit mode
quite often leads to sub-optimal and unnecessary extra code. We'll look at
this pattern as well.

Clang's BPF target supports two modes of code generation: ALU32, in which it
is capable of using lower 32-bit parts of registers, and no-ALU32, in which
only full 64-bit registers are being used. ALU32 mode somewhat mitigates the
above described problems, but not in all cases.

This patch switches all the cases in which BPF helpers return 0 or negative
error from returning int to returning long. It is shown below that such change
in definition leads to equivalent or better code. No-ALU32 mode benefits more,
but ALU32 mode doesn't degrade or still gets improved code generation.

Another class of cases switched from int to long are bpf_probe_read_str()-like
helpers, which encode successful case as non-negative values, while still
returning negative value for errors.

In all of such cases, correctness is preserved due to two's complement
encoding of negative values and the fact that all helpers return values with
32-bit absolute value. Two's complement ensures that for negative values
higher 32 bits are all ones and when truncated, leave valid negative 32-bit
value with the same value. Non-negative values have upper 32 bits set to zero
and similarly preserve value when high 32 bits are truncated. This means that
just casting to int/u32 is correct and efficient (and in ALU32 mode doesn't
require any extra shifts).

To minimize the chances of regressions, two code patterns were investigated,
as mentioned above. For both patterns, BPF assembly was analyzed in
ALU32/NO-ALU32 compiler modes, both with current 32-bit int return type and
new 64-bit long return type.

Case 1. Variable-length data reading and concatenation. This is quite
ubiquitous pattern in tracing/monitoring applications, reading data like
process's environment variables, file path, etc. In such case, many pieces of
string-like variable-length data are read into a single big buffer, and at the
end of the process, only a part of array containing actual data is sent to
user-space for further processing. This case is tested in test_varlen.c
selftest (in the next patch). Code flow is roughly as follows:

  void *payload = &sample->payload;
  u64 len;

  len = bpf_probe_read_kernel_str(payload, MAX_SZ1, &source_data1);
  if (len <= MAX_SZ1) {
      payload += len;
      sample->len1 = len;
  }
  len = bpf_probe_read_kernel_str(payload, MAX_SZ2, &source_data2);
  if (len <= MAX_SZ2) {
      payload += len;
      sample->len2 = len;
  }
  /* and so on */
  sample->total_len = payload - &sample->payload;
  /* send over, e.g., perf buffer */

There could be two variations with slightly different code generated: when len
is 64-bit integer and when it is 32-bit integer. Both variations were analysed.
BPF assembly instructions between two successive invocations of
bpf_probe_read_kernel_str() were used to check code regressions. Results are
below, followed by short analysis. Left side is using helpers with int return
type, the right one is after the switch to long.

ALU32 + INT                                ALU32 + LONG
===========                                ============

64-BIT (13 insns):                         64-BIT (10 insns):
------------------------------------       ------------------------------------
  17:   call 115                             17:   call 115
  18:   if w0 > 256 goto +9 <LBB0_4>         18:   if r0 > 256 goto +6 <LBB0_4>
  19:   w1 = w0                              19:   r1 = 0 ll
  20:   r1 <<= 32                            21:   *(u64 *)(r1 + 0) = r0
  21:   r1 s>>= 32                           22:   r6 = 0 ll
  22:   r2 = 0 ll                            24:   r6 += r0
  24:   *(u64 *)(r2 + 0) = r1              00000000000000c8 <LBB0_4>:
  25:   r6 = 0 ll                            25:   r1 = r6
  27:   r6 += r1                             26:   w2 = 256
00000000000000e0 <LBB0_4>:                   27:   r3 = 0 ll
  28:   r1 = r6                              29:   call 115
  29:   w2 = 256
  30:   r3 = 0 ll
  32:   call 115

32-BIT (11 insns):                         32-BIT (12 insns):
------------------------------------       ------------------------------------
  17:   call 115                             17:   call 115
  18:   if w0 > 256 goto +7 <LBB1_4>         18:   if w0 > 256 goto +8 <LBB1_4>
  19:   r1 = 0 ll                            19:   r1 = 0 ll
  21:   *(u32 *)(r1 + 0) = r0                21:   *(u32 *)(r1 + 0) = r0
  22:   w1 = w0                              22:   r0 <<= 32
  23:   r6 = 0 ll                            23:   r0 >>= 32
  25:   r6 += r1                             24:   r6 = 0 ll
00000000000000d0 <LBB1_4>:                   26:   r6 += r0
  26:   r1 = r6                            00000000000000d8 <LBB1_4>:
  27:   w2 = 256                             27:   r1 = r6
  28:   r3 = 0 ll                            28:   w2 = 256
  30:   call 115                             29:   r3 = 0 ll
                                             31:   call 115

In ALU32 mode, the variant using 64-bit length variable clearly wins and
avoids unnecessary zero-extension bit shifts. In practice, this is even more
important and good, because BPF code won't need to do extra checks to "prove"
that payload/len are within good bounds.

32-bit len is one instruction longer. Clang decided to do 64-to-32 casting
with two bit shifts, instead of equivalent `w1 = w0` assignment. The former
uses extra register. The latter might potentially lose some range information,
but not for 32-bit value. So in this case, verifier infers that r0 is [0, 256]
after check at 18:, and shifting 32 bits left/right keeps that range intact.
We should probably look into Clang's logic and see why it chooses bitshifts
over sub-register assignments for this.

NO-ALU32 + INT                             NO-ALU32 + LONG
==============                             ===============

64-BIT (14 insns):                         64-BIT (10 insns):
------------------------------------       ------------------------------------
  17:   call 115                             17:   call 115
  18:   r0 <<= 32                            18:   if r0 > 256 goto +6 <LBB0_4>
  19:   r1 = r0                              19:   r1 = 0 ll
  20:   r1 >>= 32                            21:   *(u64 *)(r1 + 0) = r0
  21:   if r1 > 256 goto +7 <LBB0_4>         22:   r6 = 0 ll
  22:   r0 s>>= 32                           24:   r6 += r0
  23:   r1 = 0 ll                          00000000000000c8 <LBB0_4>:
  25:   *(u64 *)(r1 + 0) = r0                25:   r1 = r6
  26:   r6 = 0 ll                            26:   r2 = 256
  28:   r6 += r0                             27:   r3 = 0 ll
00000000000000e8 <LBB0_4>:                   29:   call 115
  29:   r1 = r6
  30:   r2 = 256
  31:   r3 = 0 ll
  33:   call 115

32-BIT (13 insns):                         32-BIT (13 insns):
------------------------------------       ------------------------------------
  17:   call 115                             17:   call 115
  18:   r1 = r0                              18:   r1 = r0
  19:   r1 <<= 32                            19:   r1 <<= 32
  20:   r1 >>= 32                            20:   r1 >>= 32
  21:   if r1 > 256 goto +6 <LBB1_4>         21:   if r1 > 256 goto +6 <LBB1_4>
  22:   r2 = 0 ll                            22:   r2 = 0 ll
  24:   *(u32 *)(r2 + 0) = r0                24:   *(u32 *)(r2 + 0) = r0
  25:   r6 = 0 ll                            25:   r6 = 0 ll
  27:   r6 += r1                             27:   r6 += r1
00000000000000e0 <LBB1_4>:                 00000000000000e0 <LBB1_4>:
  28:   r1 = r6                              28:   r1 = r6
  29:   r2 = 256                             29:   r2 = 256
  30:   r3 = 0 ll                            30:   r3 = 0 ll
  32:   call 115                             32:   call 115

In NO-ALU32 mode, for the case of 64-bit len variable, Clang generates much
superior code, as expected, eliminating unnecessary bit shifts. For 32-bit
len, code is identical.

So overall, only ALU-32 32-bit len case is more-or-less equivalent and the
difference stems from internal Clang decision, rather than compiler lacking
enough information about types.

Case 2. Let's look at the simpler case of checking return result of BPF helper
for errors. The code is very simple:

  long bla;
  if (bpf_probe_read_kenerl(&bla, sizeof(bla), 0))
      return 1;
  else
      return 0;

ALU32 + CHECK (9 insns)                    ALU32 + CHECK (9 insns)
====================================       ====================================
  0:    r1 = r10                             0:    r1 = r10
  1:    r1 += -8                             1:    r1 += -8
  2:    w2 = 8                               2:    w2 = 8
  3:    r3 = 0                               3:    r3 = 0
  4:    call 113                             4:    call 113
  5:    w1 = w0                              5:    r1 = r0
  6:    w0 = 1                               6:    w0 = 1
  7:    if w1 != 0 goto +1 <LBB2_2>          7:    if r1 != 0 goto +1 <LBB2_2>
  8:    w0 = 0                               8:    w0 = 0
0000000000000048 <LBB2_2>:                 0000000000000048 <LBB2_2>:
  9:    exit                                 9:    exit

Almost identical code, the only difference is the use of full register
assignment (r1 = r0) vs half-registers (w1 = w0) in instruction #5. On 32-bit
architectures, new BPF assembly might be slightly less optimal, in theory. But
one can argue that's not a big issue, given that use of full registers is
still prevalent (e.g., for parameter passing).

NO-ALU32 + CHECK (11 insns)                NO-ALU32 + CHECK (9 insns)
====================================       ====================================
  0:    r1 = r10                             0:    r1 = r10
  1:    r1 += -8                             1:    r1 += -8
  2:    r2 = 8                               2:    r2 = 8
  3:    r3 = 0                               3:    r3 = 0
  4:    call 113                             4:    call 113
  5:    r1 = r0                              5:    r1 = r0
  6:    r1 <<= 32                            6:    r0 = 1
  7:    r1 >>= 32                            7:    if r1 != 0 goto +1 <LBB2_2>
  8:    r0 = 1                               8:    r0 = 0
  9:    if r1 != 0 goto +1 <LBB2_2>        0000000000000048 <LBB2_2>:
 10:    r0 = 0                               9:    exit
0000000000000058 <LBB2_2>:
 11:    exit

NO-ALU32 is a clear improvement, getting rid of unnecessary zero-extension bit
shifts.
Signed-off-by: default avatarAndrii Nakryiko <andriin@fb.com>
Signed-off-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200623032224.4020118-1-andriin@fb.com
      bdb7b79b
    • Alexei Starovoitov's avatar
      Merge branch 'bpftool-show-pid' · b3eece09
      Alexei Starovoitov authored 
      Andrii Nakryiko says:

====================
This patch set implements libbpf support for a second kind of special externs,
kernel symbols, in addition to existing Kconfig externs.

Right now, only untyped (const void) externs are supported, which, in
C language, allow only to take their address. In the future, with kernel BTF
getting type info about its own global and per-cpu variables, libbpf will
extend this support with BTF type info, which will allow to also directly
access variable's contents and follow its internal pointers, similarly to how
it's possible today in fentry/fexit programs.

As a first practical use of this functionality, bpftool gained ability to show
PIDs of processes that have open file descriptors for BPF map/program/link/BTF
object. It relies on iter/task_file BPF iterator program to extract this
information efficiently.

There was a bunch of bpftool refactoring (especially Makefile) necessary to
generalize bpftool's internal BPF program use. This includes generalization of
BPF skeletons support, addition of a vmlinux.h generation, extracting and
building minimal subset of bpftool for bootstrapping.

v2->v3:
- fix sec_btf_id check (Hao);

v1->v2:
- docs fixes (Quentin);
- dual GPL/BSD license for pid_inter.bpf.c (Quentin);
- NULL-init kcfg_data (Hao Luo);

rfc->v1:
- show pids, if supported by kernel, always (Alexei);
- switched iter output to binary to support showing process names;
- update man pages;
- fix few minor bugs in libbpf w.r.t. extern iteration.
====================
Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
      b3eece09
    • Andrii Nakryiko's avatar
      tools/bpftool: Add documentation and sample output for process info · 075c7766
      Andrii Nakryiko authored 
      Add statements about bpftool being able to discover process info, holding
reference to BPF map, prog, link, or BTF. Show example output as well.
Signed-off-by: default avatarAndrii Nakryiko <andriin@fb.com>
Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
Reviewed-by: default avatarQuentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/20200619231703.738941-10-andriin@fb.com
      075c7766
    • Andrii Nakryiko's avatar
      tools/bpftool: Show info for processes holding BPF map/prog/link/btf FDs · d53dee3f
      Andrii Nakryiko authored 
      Add bpf_iter-based way to find all the processes that hold open FDs against
BPF object (map, prog, link, btf). bpftool always attempts to discover this,
but will silently give up if kernel doesn't yet support bpf_iter BPF programs.
Process name and PID are emitted for each process (task group).

Sample output for each of 4 BPF objects:

$ sudo ./bpftool prog show
2694: cgroup_device  tag 8c42dee26e8cd4c2  gpl
        loaded_at 2020-06-16T15:34:32-0700  uid 0
        xlated 648B  jited 409B  memlock 4096B
        pids systemd(1)
2907: cgroup_skb  name egress  tag 9ad187367cf2b9e8  gpl
        loaded_at 2020-06-16T18:06:54-0700  uid 0
        xlated 48B  jited 59B  memlock 4096B  map_ids 2436
        btf_id 1202
        pids test_progs(2238417), test_progs(22384459)

$ sudo ./bpftool map show
2436: array  name test_cgr.bss  flags 0x400
        key 4B  value 8B  max_entries 1  memlock 8192B
        btf_id 1202
        pids test_progs(2238417), test_progs(22384459)
2445: array  name pid_iter.rodata  flags 0x480
        key 4B  value 4B  max_entries 1  memlock 8192B
        btf_id 1214  frozen
        pids bpftool(2239612)

$ sudo ./bpftool link show
61: cgroup  prog 2908
        cgroup_id 375301  attach_type egress
        pids test_progs(2238417), test_progs(22384459)
62: cgroup  prog 2908
        cgroup_id 375344  attach_type egress
        pids test_progs(2238417), test_progs(22384459)

$ sudo ./bpftool btf show
1202: size 1527B  prog_ids 2908,2907  map_ids 2436
        pids test_progs(2238417), test_progs(22384459)
1242: size 34684B
        pids bpftool(2258892)
Signed-off-by: default avatarAndrii Nakryiko <andriin@fb.com>
Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
Reviewed-by: default avatarQuentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/20200619231703.738941-9-andriin@fb.com
      d53dee3f
    • Andrii Nakryiko's avatar
      libbpf: Wrap source argument of BPF_CORE_READ macro in parentheses · bd9bedf8
      Andrii Nakryiko authored 
      Wrap source argument of BPF_CORE_READ family of macros into parentheses to
allow uses like this:

BPF_CORE_READ((struct cast_struct *)src, a, b, c);

Fixes: 7db3822a ("libbpf: Add BPF_CORE_READ/BPF_CORE_READ_INTO helpers")
Signed-off-by: default avatarAndrii Nakryiko <andriin@fb.com>
Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200619231703.738941-8-andriin@fb.com
      bd9bedf8
    • Andrii Nakryiko's avatar
      tools/bpftool: Generalize BPF skeleton support and generate vmlinux.h · 05aca6da
      Andrii Nakryiko authored 
      Adapt Makefile to support BPF skeleton generation beyond single profiler.bpf.c
case. Also add vmlinux.h generation and switch profiler.bpf.c to use it.

clang-bpf-global-var feature is extended and renamed to clang-bpf-co-re to
check for support of preserve_access_index attribute, which, together with BTF
for global variables, is the minimum requirement for modern BPF programs.
Signed-off-by: default avatarAndrii Nakryiko <andriin@fb.com>
Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
Reviewed-by: default avatarQuentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/20200619231703.738941-7-andriin@fb.com
      05aca6da
    • Andrii Nakryiko's avatar
      tools/bpftool: Minimize bootstrap bpftool · 16e9b187
      Andrii Nakryiko authored 
      Build minimal "bootstrap mode" bpftool to enable skeleton (and, later,
vmlinux.h generation), instead of building almost complete, but slightly
different (w/o skeletons, etc) bpftool to bootstrap complete bpftool build.

Current approach doesn't scale well (engineering-wise) when adding more BPF
programs to bpftool and other complicated functionality, as it requires
constant adjusting of the code to work in both bootstrapped mode and normal
mode.

So it's better to build only minimal bpftool version that supports only BPF
skeleton code generation and BTF-to-C conversion. Thankfully, this is quite
easy to accomplish due to internal modularity of bpftool commands. This will
also allow to keep adding new functionality to bpftool in general, without the
need to care about bootstrap mode for those new parts of bpftool.
Signed-off-by: default avatarAndrii Nakryiko <andriin@fb.com>
Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
Reviewed-by: default avatarQuentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/20200619231703.738941-6-andriin@fb.com
      16e9b187
    • Andrii Nakryiko's avatar
      tools/bpftool: Move map/prog parsing logic into common · a479b8ce
      Andrii Nakryiko authored 
      Move functions that parse map and prog by id/tag/name/etc outside of
map.c/prog.c, respectively. These functions are used outside of those files
and are generic enough to be in common. This also makes heavy-weight map.c and
prog.c more decoupled from the rest of bpftool files and facilitates more
lightweight bootstrap bpftool variant.
Signed-off-by: default avatarAndrii Nakryiko <andriin@fb.com>
Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
Reviewed-by: default avatarQuentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/20200619231703.738941-5-andriin@fb.com
      a479b8ce
    • Andrii Nakryiko's avatar
      selftests/bpf: Add __ksym extern selftest · b7ddfab2
      Andrii Nakryiko authored 
      Validate libbpf is able to handle weak and strong kernel symbol externs in BPF
code correctly.
Signed-off-by: default avatarAndrii Nakryiko <andriin@fb.com>
Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
Reviewed-by: default avatarHao Luo <haoluo@google.com>
Link: https://lore.kernel.org/bpf/20200619231703.738941-4-andriin@fb.com
      b7ddfab2
    • Andrii Nakryiko's avatar
      libbpf: Add support for extracting kernel symbol addresses · 1c0c7074
      Andrii Nakryiko authored 
      Add support for another (in addition to existing Kconfig) special kind of
externs in BPF code, kernel symbol externs. Such externs allow BPF code to
"know" kernel symbol address and either use it for comparisons with kernel
data structures (e.g., struct file's f_op pointer, to distinguish different
kinds of file), or, with the help of bpf_probe_user_kernel(), to follow
pointers and read data from global variables. Kernel symbol addresses are
found through /proc/kallsyms, which should be present in the system.

Currently, such kernel symbol variables are typeless: they have to be defined
as `extern const void <symbol>` and the only operation you can do (in C code)
with them is to take its address. Such extern should reside in a special
section '.ksyms'. bpf_helpers.h header provides __ksym macro for this. Strong
vs weak semantics stays the same as with Kconfig externs. If symbol is not
found in /proc/kallsyms, this will be a failure for strong (non-weak) extern,
but will be defaulted to 0 for weak externs.

If the same symbol is defined multiple times in /proc/kallsyms, then it will
be error if any of the associated addresses differs. In that case, address is
ambiguous, so libbpf falls on the side of caution, rather than confusing user
with randomly chosen address.

In the future, once kernel is extended with variables BTF information, such
ksym externs will be supported in a typed version, which will allow BPF
program to read variable's contents directly, similarly to how it's done for
fentry/fexit input arguments.
Signed-off-by: default avatarAndrii Nakryiko <andriin@fb.com>
Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
Reviewed-by: default avatarHao Luo <haoluo@google.com>
Link: https://lore.kernel.org/bpf/20200619231703.738941-3-andriin@fb.com
      1c0c7074
    • Andrii Nakryiko's avatar
      libbpf: Generalize libbpf externs support · 2e33efe3
      Andrii Nakryiko authored 
      Switch existing Kconfig externs to be just one of few possible kinds of more
generic externs. This refactoring is in preparation for ksymbol extern
support, added in the follow up patch. There are no functional changes
intended.
Signed-off-by: default avatarAndrii Nakryiko <andriin@fb.com>
Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
Reviewed-by: default avatarHao Luo <haoluo@google.com>
Link: https://lore.kernel.org/bpf/20200619231703.738941-2-andriin@fb.com
      2e33efe3
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