[PATCH v2 2/5] objtool: Fix ORC unwinding in non-JIT BPF generated code

[Josh Poimboeuf <jpoimboe@redhat.com>]

Objtool currently ignores ___bpf_prog_run() because it doesn't
understand the jump table.  This results in the ORC unwinder not being
able to unwind through non-JIT BPF code.

Luckily, the BPF jump table resembles a GCC switch jump table, which
objtool already knows how to read.

Add generic support for reading any static local jump table array named
"jump_table", and rename the BPF variable accordingly, so objtool can
generate ORC data for ___bpf_prog_run().

Fixes: d15d356887e7 ("perf/x86: Make perf callchains work without CONFIG_FRAME_POINTER")
Reported-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 kernel/bpf/core.c     |  5 ++---
 tools/objtool/check.c | 16 ++++++++++++++--
 2 files changed, 16 insertions(+), 5 deletions(-)

diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index 7c473f208a10..aa546ef7dbdc 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -1299,7 +1299,7 @@ static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn, u64 *stack)
 {
 #define BPF_INSN_2_LBL(x, y)    [BPF_##x | BPF_##y] = &&x##_##y
 #define BPF_INSN_3_LBL(x, y, z) [BPF_##x | BPF_##y | BPF_##z] = &&x##_##y##_##z
-	static const void *jumptable[256] = {
+	static const void *jump_table[256] = {
 		[0 ... 255] = &&default_label,
 		/* Now overwrite non-defaults ... */
 		BPF_INSN_MAP(BPF_INSN_2_LBL, BPF_INSN_3_LBL),
@@ -1315,7 +1315,7 @@ static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn, u64 *stack)
 #define CONT_JMP ({ insn++; goto select_insn; })
 
 select_insn:
-	goto *jumptable[insn->code];
+	goto *jump_table[insn->code];
 
 	/* ALU */
 #define ALU(OPCODE, OP)			\
@@ -1558,7 +1558,6 @@ static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn, u64 *stack)
 		BUG_ON(1);
 		return 0;
 }
-STACK_FRAME_NON_STANDARD(___bpf_prog_run); /* jump table */
 
 #define PROG_NAME(stack_size) __bpf_prog_run##stack_size
 #define DEFINE_BPF_PROG_RUN(stack_size) \
diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 172f99195726..8341c2fff14f 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -18,6 +18,8 @@
 
 #define FAKE_JUMP_OFFSET -1
 
+#define JUMP_TABLE_SYM_PREFIX "jump_table."
+
 struct alternative {
 	struct list_head list;
 	struct instruction *insn;
@@ -997,6 +999,7 @@ static struct rela *find_switch_table(struct objtool_file *file,
 	struct instruction *orig_insn = insn;
 	struct section *rodata_sec;
 	unsigned long table_offset;
+	struct symbol *sym;
 
 	/*
 	 * Backward search using the @first_jump_src links, these help avoid
@@ -1035,9 +1038,18 @@ static struct rela *find_switch_table(struct objtool_file *file,
 
 		/*
 		 * Make sure the .rodata address isn't associated with a
-		 * symbol.  gcc jump tables are anonymous data.
+		 * symbol.  GCC jump tables are anonymous data.
+		 *
+		 * Also support C jump tables which are in the same format as
+		 * switch jump tables.  Each jump table should be a static
+		 * local const array named "jump_table" for objtool to
+		 * recognize it.  Note: GCC will add a numbered suffix to the
+		 * ELF symbol name, like "jump_table.12345", which it does for
+		 * all static local variables.
 		 */
-		if (find_symbol_containing(rodata_sec, table_offset))
+		sym = find_symbol_containing(rodata_sec, table_offset);
+		if (sym && strncmp(sym->name, JUMP_TABLE_SYM_PREFIX,
+				   strlen(JUMP_TABLE_SYM_PREFIX)))
 			continue;
 
 		rodata_rela = find_rela_by_dest(rodata_sec, table_offset);
-- 
2.20.1