Subject: [PATCH 1/3] arm64: vdso: Don't prefix sigreturn trampoline with a BTI C instruction

From: Will Deacon <[email-masked]>
Date: Tue, 19 May 2020 13:18:16 +0100

For better or worse, GDB relies on the exact instruction sequence in the
VDSO sigreturn trampoline in order to unwind from signals correctly.
Commit 714a8d02ca4d ("arm64: asm: Override SYM_FUNC_START when building
the kernel with BTI") unfortunately added a BTI C instruction to the
start of __kernel_rt_sigreturn, which breaks this check. Thankfully,
it's also not required, since the trampoline is called from a RET
instruction when returning from the signal handler

Remove the unnecessary BTI C instruction from __kernel_rt_sigreturn.

Cc: Dave Martin <[email-masked]>
Cc: Mark Brown <[email-masked]>
Cc: Daniel Kiss <[email-masked]>
Cc: Tamas Zsoldos <[email-masked]>
Fixes: 714a8d02ca4d ("arm64: asm: Override SYM_FUNC_START when building the kernel with BTI")
Signed-off-by: Will Deacon <[email-masked]>
---
 arch/arm64/kernel/vdso/sigreturn.S | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kernel/vdso/sigreturn.S b/arch/arm64/kernel/vdso/sigreturn.S
index 3fb13b81f780..83ac284dae79 100644
--- a/arch/arm64/kernel/vdso/sigreturn.S
+++ b/arch/arm64/kernel/vdso/sigreturn.S
@@ -15,7 +15,14 @@
 	.text
 
 	nop
-SYM_FUNC_START(__kernel_rt_sigreturn)
+/*
+ * GDB relies on being able to identify the sigreturn instruction sequence to
+ * unwind from signal handlers. We cannot, therefore, use SYM_FUNC_START()
+ * here, as it will emit a BTI C instruction and break the unwinder. Thankfully,
+ * this function is only ever called from a RET and so omitting the landing pad
+ * is perfectly fine.
+ */
+SYM_START(__kernel_rt_sigreturn, SYM_L_GLOBAL, SYM_A_ALIGN)
 	.cfi_startproc
 	.cfi_signal_frame
 	.cfi_def_cfa	x29, 0
-- 
2.26.2.761.g0e0b3e54be-goog

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