bpf: extend stackmap to save binary_build_id+offset instead of address

Currently, bpf stackmap store address for each entry in the call trace.
To map these addresses to user space files, it is necessary to maintain
the mapping from these virtual address to symbols in the binary. Usually,
the user space profiler (such as perf) has to scan /proc/pid/maps at the
beginning of profiling, and monitor mmap2() calls afterwards. Given the
cost of maintaining the address map, this solution is not practical for
system wide profiling that is always on.

This patch tries to solve this problem with a variation of stackmap. This
variation is enabled by flag BPF_F_STACK_BUILD_ID. Instead of storing
addresses, the variation stores ELF file build_id + offset.

Build ID is a 20-byte unique identifier for ELF files. The following
command shows the Build ID of /bin/bash:

  [user@]$ readelf -n /bin/bash
  ...
    Build ID: XXXXXXXXXX
  ...

With BPF_F_STACK_BUILD_ID, bpf_get_stackid() tries to parse Build ID
for each entry in the call trace, and translate it into the following
struct:

  struct bpf_stack_build_id_offset {
          __s32           status;
          unsigned char   build_id[BPF_BUILD_ID_SIZE];
          union {
                  __u64   offset;
                  __u64   ip;
          };
  };

The search of build_id is limited to the first page of the file, and this
page should be in page cache. Otherwise, we fallback to store ip for this
entry (ip field in struct bpf_stack_build_id_offset). This requires the
build_id to be stored in the first page. A quick survey of binary and
dynamic library files in a few different systems shows that almost all
binary and dynamic library files have build_id in the first page.

Build_id is only meaningful for user stack. If a kernel stack is added to
a stackmap with BPF_F_STACK_BUILD_ID, it will automatically fallback to
only store ip (status == BPF_STACK_BUILD_ID_IP). Similarly, if build_id
lookup failed for some reason, it will also fallback to store ip.

User space can access struct bpf_stack_build_id_offset with bpf
syscall BPF_MAP_LOOKUP_ELEM. It is necessary for user space to
maintain mapping from build id to binary files. This mostly static
mapping is much easier to maintain than per process address maps.

Note: Stackmap with build_id only works in non-nmi context at this time.
This is because we need to take mm->mmap_sem for find_vma(). If this
changes, we would like to allow build_id lookup in nmi context.
Signed-off-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

include/uapi/linux/bpf.h

@@ -231,6 +231,28 @@ enum bpf_attach_type {
 #define BPF_F_RDONLY		(1U << 3)
 #define BPF_F_WRONLY		(1U << 4)
 
+/* Flag for stack_map, store build_id+offset instead of pointer */
+#define BPF_F_STACK_BUILD_ID	(1U << 5)
+
+enum bpf_stack_build_id_status {
+	/* user space need an empty entry to identify end of a trace */
+	BPF_STACK_BUILD_ID_EMPTY = 0,
+	/* with valid build_id and offset */
+	BPF_STACK_BUILD_ID_VALID = 1,
+	/* couldn't get build_id, fallback to ip */
+	BPF_STACK_BUILD_ID_IP = 2,
+};
+
+#define BPF_BUILD_ID_SIZE 20
+struct bpf_stack_build_id {
+	__s32		status;
+	unsigned char	build_id[BPF_BUILD_ID_SIZE];
+	union {
+		__u64	offset;
+		__u64	ip;
+	};
+};
+
 union bpf_attr {
 	struct { /* anonymous struct used by BPF_MAP_CREATE command */
 		__u32	map_type;	/* one of enum bpf_map_type */

kernel/bpf/stackmap.c

@@ -9,16 +9,19 @@
 #include <linux/filter.h>
 #include <linux/stacktrace.h>
 #include <linux/perf_event.h>
+#include <linux/elf.h>
+#include <linux/pagemap.h>
 #include "percpu_freelist.h"
 
 #define STACK_CREATE_FLAG_MASK					\
-	(BPF_F_NUMA_NODE | BPF_F_RDONLY | BPF_F_WRONLY)
+	(BPF_F_NUMA_NODE | BPF_F_RDONLY | BPF_F_WRONLY |	\
+	 BPF_F_STACK_BUILD_ID)
 
 struct stack_map_bucket {
 	struct pcpu_freelist_node fnode;
 	u32 hash;
 	u32 nr;
-	u64 ip[];
+	u64 data[];
 };
 
 struct bpf_stack_map {
@@ -29,6 +32,17 @@ struct bpf_stack_map {
 	struct stack_map_bucket *buckets[];
 };
 
+static inline bool stack_map_use_build_id(struct bpf_map *map)
+{
+	return (map->map_flags & BPF_F_STACK_BUILD_ID);
+}
+
+static inline int stack_map_data_size(struct bpf_map *map)
+{
+	return stack_map_use_build_id(map) ?
+		sizeof(struct bpf_stack_build_id) : sizeof(u64);
+}
+
 static int prealloc_elems_and_freelist(struct bpf_stack_map *smap)
 {
 	u32 elem_size = sizeof(struct stack_map_bucket) + smap->map.value_size;
@@ -68,8 +82,16 @@ static struct bpf_map *stack_map_alloc(union bpf_attr *attr)
 
 	/* check sanity of attributes */
 	if (attr->max_entries == 0 || attr->key_size != 4 ||
-	    value_size < 8 || value_size % 8 ||
-	    value_size / 8 > sysctl_perf_event_max_stack)
+	    value_size < 8 || value_size % 8)
+		return ERR_PTR(-EINVAL);
+
+	BUILD_BUG_ON(sizeof(struct bpf_stack_build_id) % sizeof(u64));
+	if (attr->map_flags & BPF_F_STACK_BUILD_ID) {
+		if (value_size % sizeof(struct bpf_stack_build_id) ||
+		    value_size / sizeof(struct bpf_stack_build_id)
+		    > sysctl_perf_event_max_stack)
+			return ERR_PTR(-EINVAL);
+	} else if (value_size / 8 > sysctl_perf_event_max_stack)
 		return ERR_PTR(-EINVAL);
 
 	/* hash table size must be power of 2 */
@@ -114,13 +136,184 @@ static struct bpf_map *stack_map_alloc(union bpf_attr *attr)
 	return ERR_PTR(err);
 }
 
+#define BPF_BUILD_ID 3
+/*
+ * Parse build id from the note segment. This logic can be shared between
+ * 32-bit and 64-bit system, because Elf32_Nhdr and Elf64_Nhdr are
+ * identical.
+ */
+static inline int stack_map_parse_build_id(void *page_addr,
+					   unsigned char *build_id,
+					   void *note_start,
+					   Elf32_Word note_size)
+{
+	Elf32_Word note_offs = 0, new_offs;
+
+	/* check for overflow */
+	if (note_start < page_addr || note_start + note_size < note_start)
+		return -EINVAL;
+
+	/* only supports note that fits in the first page */
+	if (note_start + note_size > page_addr + PAGE_SIZE)
+		return -EINVAL;
+
+	while (note_offs + sizeof(Elf32_Nhdr) < note_size) {
+		Elf32_Nhdr *nhdr = (Elf32_Nhdr *)(note_start + note_offs);
+
+		if (nhdr->n_type == BPF_BUILD_ID &&
+		    nhdr->n_namesz == sizeof("GNU") &&
+		    nhdr->n_descsz == BPF_BUILD_ID_SIZE) {
+			memcpy(build_id,
+			       note_start + note_offs +
+			       ALIGN(sizeof("GNU"), 4) + sizeof(Elf32_Nhdr),
+			       BPF_BUILD_ID_SIZE);
+			return 0;
+		}
+		new_offs = note_offs + sizeof(Elf32_Nhdr) +
+			ALIGN(nhdr->n_namesz, 4) + ALIGN(nhdr->n_descsz, 4);
+		if (new_offs <= note_offs)  /* overflow */
+			break;
+		note_offs = new_offs;
+	}
+	return -EINVAL;
+}
+
+/* Parse build ID from 32-bit ELF */
+static int stack_map_get_build_id_32(void *page_addr,
+				     unsigned char *build_id)
+{
+	Elf32_Ehdr *ehdr = (Elf32_Ehdr *)page_addr;
+	Elf32_Phdr *phdr;
+	int i;
+
+	/* only supports phdr that fits in one page */
+	if (ehdr->e_phnum >
+	    (PAGE_SIZE - sizeof(Elf32_Ehdr)) / sizeof(Elf32_Phdr))
+		return -EINVAL;
+
+	phdr = (Elf32_Phdr *)(page_addr + sizeof(Elf32_Ehdr));
+
+	for (i = 0; i < ehdr->e_phnum; ++i)
+		if (phdr[i].p_type == PT_NOTE)
+			return stack_map_parse_build_id(page_addr, build_id,
+					page_addr + phdr[i].p_offset,
+					phdr[i].p_filesz);
+	return -EINVAL;
+}
+
+/* Parse build ID from 64-bit ELF */
+static int stack_map_get_build_id_64(void *page_addr,
+				     unsigned char *build_id)
+{
+	Elf64_Ehdr *ehdr = (Elf64_Ehdr *)page_addr;
+	Elf64_Phdr *phdr;
+	int i;
+
+	/* only supports phdr that fits in one page */
+	if (ehdr->e_phnum >
+	    (PAGE_SIZE - sizeof(Elf64_Ehdr)) / sizeof(Elf64_Phdr))
+		return -EINVAL;
+
+	phdr = (Elf64_Phdr *)(page_addr + sizeof(Elf64_Ehdr));
+
+	for (i = 0; i < ehdr->e_phnum; ++i)
+		if (phdr[i].p_type == PT_NOTE)
+			return stack_map_parse_build_id(page_addr, build_id,
+					page_addr + phdr[i].p_offset,
+					phdr[i].p_filesz);
+	return -EINVAL;
+}
+
+/* Parse build ID of ELF file mapped to vma */
+static int stack_map_get_build_id(struct vm_area_struct *vma,
+				  unsigned char *build_id)
+{
+	Elf32_Ehdr *ehdr;
+	struct page *page;
+	void *page_addr;
+	int ret;
+
+	/* only works for page backed storage  */
+	if (!vma->vm_file)
+		return -EINVAL;
+
+	page = find_get_page(vma->vm_file->f_mapping, 0);
+	if (!page)
+		return -EFAULT;	/* page not mapped */
+
+	ret = -EINVAL;
+	page_addr = page_address(page);
+	ehdr = (Elf32_Ehdr *)page_addr;
+
+	/* compare magic x7f "ELF" */
+	if (memcmp(ehdr->e_ident, ELFMAG, SELFMAG) != 0)
+		goto out;
+
+	/* only support executable file and shared object file */
+	if (ehdr->e_type != ET_EXEC && ehdr->e_type != ET_DYN)
+		goto out;
+
+	if (ehdr->e_ident[EI_CLASS] == ELFCLASS32)
+		ret = stack_map_get_build_id_32(page_addr, build_id);
+	else if (ehdr->e_ident[EI_CLASS] == ELFCLASS64)
+		ret = stack_map_get_build_id_64(page_addr, build_id);
+out:
+	put_page(page);
+	return ret;
+}
+
+static void stack_map_get_build_id_offset(struct bpf_map *map,
+					  struct stack_map_bucket *bucket,
+					  u64 *ips, u32 trace_nr, bool user)
+{
+	int i;
+	struct vm_area_struct *vma;
+	struct bpf_stack_build_id *id_offs;
+
+	bucket->nr = trace_nr;
+	id_offs = (struct bpf_stack_build_id *)bucket->data;
+
+	/*
+	 * We cannot do up_read() in nmi context, so build_id lookup is
+	 * only supported for non-nmi events. If at some point, it is
+	 * possible to run find_vma() without taking the semaphore, we
+	 * would like to allow build_id lookup in nmi context.
+	 *
+	 * Same fallback is used for kernel stack (!user) on a stackmap
+	 * with build_id.
+	 */
+	if (!user || !current || !current->mm || in_nmi() ||
+	    down_read_trylock(&current->mm->mmap_sem) == 0) {
+		/* cannot access current->mm, fall back to ips */
+		for (i = 0; i < trace_nr; i++) {
+			id_offs[i].status = BPF_STACK_BUILD_ID_IP;
+			id_offs[i].ip = ips[i];
+		}
+		return;
+	}
+
+	for (i = 0; i < trace_nr; i++) {
+		vma = find_vma(current->mm, ips[i]);
+		if (!vma || stack_map_get_build_id(vma, id_offs[i].build_id)) {
+			/* per entry fall back to ips */
+			id_offs[i].status = BPF_STACK_BUILD_ID_IP;
+			id_offs[i].ip = ips[i];
+			continue;
+		}
+		id_offs[i].offset = (vma->vm_pgoff << PAGE_SHIFT) + ips[i]
+			- vma->vm_start;
+		id_offs[i].status = BPF_STACK_BUILD_ID_VALID;
+	}
+	up_read(&current->mm->mmap_sem);
+}
+
 BPF_CALL_3(bpf_get_stackid, struct pt_regs *, regs, struct bpf_map *, map,
 	   u64, flags)
 {
 	struct bpf_stack_map *smap = container_of(map, struct bpf_stack_map, map);
 	struct perf_callchain_entry *trace;
 	struct stack_map_bucket *bucket, *new_bucket, *old_bucket;
-	u32 max_depth = map->value_size / 8;
+	u32 max_depth = map->value_size / stack_map_data_size(map);
 	/* stack_map_alloc() checks that max_depth <= sysctl_perf_event_max_stack */
 	u32 init_nr = sysctl_perf_event_max_stack - max_depth;
 	u32 skip = flags & BPF_F_SKIP_FIELD_MASK;
@@ -128,6 +321,7 @@ BPF_CALL_3(bpf_get_stackid, struct pt_regs *, regs, struct bpf_map *, map,
 	bool user = flags & BPF_F_USER_STACK;
 	bool kernel = !user;
 	u64 *ips;
+	bool hash_matches;
 
 	if (unlikely(flags & ~(BPF_F_SKIP_FIELD_MASK | BPF_F_USER_STACK |
 			       BPF_F_FAST_STACK_CMP | BPF_F_REUSE_STACKID)))
@@ -156,15 +350,33 @@ BPF_CALL_3(bpf_get_stackid, struct pt_regs *, regs, struct bpf_map *, map,
 	id = hash & (smap->n_buckets - 1);
 	bucket = READ_ONCE(smap->buckets[id]);
 
-	if (bucket && bucket->hash == hash) {
-		if (flags & BPF_F_FAST_STACK_CMP)
+	hash_matches = bucket && bucket->hash == hash;
+	/* fast cmp */
+	if (hash_matches && flags & BPF_F_FAST_STACK_CMP)
 		return id;
-		if (bucket->nr == trace_nr &&
-		    memcmp(bucket->ip, ips, trace_len) == 0)
+
+	if (stack_map_use_build_id(map)) {
+		/* for build_id+offset, pop a bucket before slow cmp */
+		new_bucket = (struct stack_map_bucket *)
+			pcpu_freelist_pop(&smap->freelist);
+		if (unlikely(!new_bucket))
+			return -ENOMEM;
+		stack_map_get_build_id_offset(map, new_bucket, ips,
+					      trace_nr, user);
+		trace_len = trace_nr * sizeof(struct bpf_stack_build_id);
+		if (hash_matches && bucket->nr == trace_nr &&
+		    memcmp(bucket->data, new_bucket->data, trace_len) == 0) {
+			pcpu_freelist_push(&smap->freelist, &new_bucket->fnode);
 			return id;
 		}
-
-	/* this call stack is not in the map, try to add it */
+		if (bucket && !(flags & BPF_F_REUSE_STACKID)) {
+			pcpu_freelist_push(&smap->freelist, &new_bucket->fnode);
+			return -EEXIST;
+		}
+	} else {
+		if (hash_matches && bucket->nr == trace_nr &&
+		    memcmp(bucket->data, ips, trace_len) == 0)
+			return id;
 		if (bucket && !(flags & BPF_F_REUSE_STACKID))
 			return -EEXIST;
 
@@ -172,8 +384,9 @@ BPF_CALL_3(bpf_get_stackid, struct pt_regs *, regs, struct bpf_map *, map,
 			pcpu_freelist_pop(&smap->freelist);
 		if (unlikely(!new_bucket))
 			return -ENOMEM;
+		memcpy(new_bucket->data, ips, trace_len);
+	}
 
-	memcpy(new_bucket->ip, ips, trace_len);
 	new_bucket->hash = hash;
 	new_bucket->nr = trace_nr;
 
@@ -212,8 +425,8 @@ int bpf_stackmap_copy(struct bpf_map *map, void *key, void *value)
 	if (!bucket)
 		return -ENOENT;
 
-	trace_len = bucket->nr * sizeof(u64);
-	memcpy(value, bucket->ip, trace_len);
+	trace_len = bucket->nr * stack_map_data_size(map);
+	memcpy(value, bucket->data, trace_len);
 	memset(value + trace_len, 0, map->value_size - trace_len);
 
 	old_bucket = xchg(&smap->buckets[id], bucket);