libbpf: Support CO-RE relocations for LDX/ST/STX instructions

Clang patch [0] enables emitting relocatable generic ALU/ALU64 instructions
(i.e, shifts and arithmetic operations), as well as generic load/store
instructions. The former ones are already supported by libbpf as is. This
patch adds further support for load/store instructions. Relocatable field
offset is encoded in BPF instruction's 16-bit offset section and are adjusted
by libbpf based on target kernel BTF.

These Clang changes and corresponding libbpf changes allow for more succinct
generated BPF code by encoding relocatable field reads as a single
ST/LDX/STX instruction. It also enables relocatable access to BPF context.
Previously, if context struct (e.g., __sk_buff) was accessed with CO-RE
relocations (e.g., due to preserve_access_index attribute), it would be
rejected by BPF verifier due to modified context pointer dereference. With
Clang patch, such context accesses are both relocatable and have a fixed
offset from the point of view of BPF verifier.

  [0] https://reviews.llvm.org/D71790Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20191223180305.86417-1-andriin@fb.com

tools/lib/bpf/libbpf.c

@@ -18,6 +18,7 @@
 #include <stdarg.h>
 #include <libgen.h>
 #include <inttypes.h>
+#include <limits.h>
 #include <string.h>
 #include <unistd.h>
 #include <endian.h>
@@ -3810,11 +3811,13 @@ static int bpf_core_reloc_insn(struct bpf_program *prog,
 	insn = &prog->insns[insn_idx];
 	class = BPF_CLASS(insn->code);
 
-	if (class == BPF_ALU || class == BPF_ALU64) {
+	switch (class) {
+	case BPF_ALU:
+	case BPF_ALU64:
 		if (BPF_SRC(insn->code) != BPF_K)
 			return -EINVAL;
 		if (!failed && validate && insn->imm != orig_val) {
-			pr_warn("prog '%s': unexpected insn #%d value: got %u, exp %u -> %u\n",
+			pr_warn("prog '%s': unexpected insn #%d (ALU/ALU64) value: got %u, exp %u -> %u\n",
 				bpf_program__title(prog, false), insn_idx,
 				insn->imm, orig_val, new_val);
 			return -EINVAL;
@@ -3824,7 +3827,29 @@ static int bpf_core_reloc_insn(struct bpf_program *prog,
 		pr_debug("prog '%s': patched insn #%d (ALU/ALU64)%s imm %u -> %u\n",
 			 bpf_program__title(prog, false), insn_idx,
 			 failed ? " w/ failed reloc" : "", orig_val, new_val);
-	} else {
+		break;
+	case BPF_LDX:
+	case BPF_ST:
+	case BPF_STX:
+		if (!failed && validate && insn->off != orig_val) {
+			pr_warn("prog '%s': unexpected insn #%d (LD/LDX/ST/STX) value: got %u, exp %u -> %u\n",
+				bpf_program__title(prog, false), insn_idx,
+				insn->off, orig_val, new_val);
+			return -EINVAL;
+		}
+		if (new_val > SHRT_MAX) {
+			pr_warn("prog '%s': insn #%d (LD/LDX/ST/STX) value too big: %u\n",
+				bpf_program__title(prog, false), insn_idx,
+				new_val);
+			return -ERANGE;
+		}
+		orig_val = insn->off;
+		insn->off = new_val;
+		pr_debug("prog '%s': patched insn #%d (LD/LDX/ST/STX)%s off %u -> %u\n",
+			 bpf_program__title(prog, false), insn_idx,
+			 failed ? " w/ failed reloc" : "", orig_val, new_val);
+		break;
+	default:
 		pr_warn("prog '%s': trying to relocate unrecognized insn #%d, code:%x, src:%x, dst:%x, off:%x, imm:%x\n",
 			bpf_program__title(prog, false),
 			insn_idx, insn->code, insn->src_reg, insn->dst_reg,