[PATCH 1/3] objtool/x86: Move the cfi->regs access after the NULL check

[Leizhen Zhang <lzsx618@gmail.com>]

init_orc_entry() takes &cfi->regs[CFI_BP] before it checks whether cfi
is NULL, and a NULL cfi is an expected case: the comment right below
says it happens for unreachable nops/traps and for
STACK_FRAME_NON_STANDARD functions.

Forming a member address through a NULL pointer is undefined behaviour,
and since the dereference comes first the compiler is free to assume
cfi != NULL and drop the check that follows.  An x86_64 defconfig build
of v7.4-rc2 with objtool built with -fsanitize=undefined hits it in
every objtool run, on vmlinux.o and on each module, all through
orc_create():

  arch/x86/orc.c:11:18: runtime error: member access within null pointer of type 'struct cfi_state'
    #0 init_orc_entry arch/x86/orc.c:11
    #1 orc_create tools/objtool/orc_gen.c:72
    #2 check tools/objtool/check.c:4939

Take the address only after the NULL check.  Running objtool with and
without this patch on the same defconfig vmlinux.o gives byte-identical
output.

The report was found with UBSan, and the analysis and fix were
prepared with the help of an AI coding assistant; I reviewed and
tested the result.

Fixes: 8b946cc38e06 ("objtool: Introduce CFI hash")
Signed-off-by: Leizhen Zhang <lzsx618@gmail.com>
---
 tools/objtool/arch/x86/orc.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/tools/objtool/arch/x86/orc.c b/tools/objtool/arch/x86/orc.c
index eff078ecc945..b688afcc4ffc 100644
--- a/tools/objtool/arch/x86/orc.c
+++ b/tools/objtool/arch/x86/orc.c
@@ -8,7 +8,7 @@
 
 int init_orc_entry(struct orc_entry *orc, struct cfi_state *cfi, struct instruction *insn)
 {
-	struct cfi_reg *bp = &cfi->regs[CFI_BP];
+	struct cfi_reg *bp;
 
 	memset(orc, 0, sizeof(*orc));
 
@@ -22,6 +22,8 @@ int init_orc_entry(struct orc_entry *orc, struct cfi_state *cfi, struct instruct
 		return 0;
 	}
 
+	bp = &cfi->regs[CFI_BP];
+
 	switch (cfi->type) {
 	case UNWIND_HINT_TYPE_UNDEFINED:
 		orc->type = ORC_TYPE_UNDEFINED;
-- 
2.34.1