[PATCH] ARM: kretprobes: fix out-of-range literal issue in naked asm routine

__kretprobe_trampoline is a naked C function implemented in asm, and the
first thing it does is take its own address and record it in the frame
record, so that a backtrace will properly traverse its stackframe.

As it turns out, using 'LDR <reg>, =<symbol>' notation is problematic
here, as it relies on a literal pool to be emitted sufficiently close to
the LDR instruction, and since the compiler is not aware of this need,
this results in assembler errors in some cases.

Since the address taken is guaranteed to be the address of the
instruction itself, just use ADR instead.

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Reported-by: kernel test robot <redacted>
Fixes: 7e9bf33b8124 ("ARM: kprobes: Make a frame pointer on __kretprobe_trampoline")
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 arch/arm/probes/kprobes/core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm/probes/kprobes/core.c b/arch/arm/probes/kprobes/core.c
index 9090c3a74dcc..068639a777c8 100644
--- a/arch/arm/probes/kprobes/core.c
+++ b/arch/arm/probes/kprobes/core.c

@@ -377,7 +377,7 @@ void __naked __kprobes __kretprobe_trampoline(void)
 {
 	__asm__ __volatile__ (
 #ifdef CONFIG_FRAME_POINTER
-		"ldr	lr, =__kretprobe_trampoline	\n\t"
+		"adr	lr, .			\n\t"
 	/* __kretprobe_trampoline makes a framepointer on pt_regs. */
 #ifdef CONFIG_CC_IS_CLANG
 		"stmdb	sp, {sp, lr, pc}	\n\t"

-- 
2.30.2


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On Fri, 25 Mar 2022 09:03:02 +0100
Ard Biesheuvel [off-list ref] wrote:

__kretprobe_trampoline is a naked C function implemented in asm, and the
first thing it does is take its own address and record it in the frame
record, so that a backtrace will properly traverse its stackframe.

As it turns out, using 'LDR <reg>, =<symbol>' notation is problematic
here, as it relies on a literal pool to be emitted sufficiently close to
the LDR instruction, and since the compiler is not aware of this need,
this results in assembler errors in some cases.

Since the address taken is guaranteed to be the address of the
instruction itself, just use ADR instead.

Ah, indeed :)

This looks good to me.

Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
Tested-by: Masami Hiramatsu <mhiramat@kernel.org>

Thank you very much!

-- 
Masami Hiramatsu [off-list ref]

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