Subject: [PATCH v3 1/2] arm64: vdso: Don't prefix sigreturn trampoline with a BTI C instruction

From: Will Deacon <[email-masked]>
Date: Wed, 20 May 2020 18:55:38 +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 c91db232da48 ("arm64: vdso: Convert to modern assembler annotations")
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,
and do the same for the 32-bit VDSO as well for good measure.

Cc: Daniel Kiss <[email-masked]>
Cc: Tamas Zsoldos <[email-masked]>
Reviewed-by: Dave Martin <[email-masked]>
Reviewed-by: Mark Brown <[email-masked]>
Fixes: c91db232da48 ("arm64: vdso: Convert to modern assembler annotations")
Signed-off-by: Will Deacon <[email-masked]>
---
 arch/arm64/include/asm/linkage.h     |  6 +++---
 arch/arm64/kernel/vdso/sigreturn.S   | 11 +++++++++--
 arch/arm64/kernel/vdso32/sigreturn.S | 16 ++++++++--------
 3 files changed, 20 insertions(+), 13 deletions(-)

diff --git a/arch/arm64/include/asm/linkage.h b/arch/arm64/include/asm/linkage.h
index b5a7998a6b2a..81fefd2a1d02 100644
--- a/arch/arm64/include/asm/linkage.h
+++ b/arch/arm64/include/asm/linkage.h
@@ -15,9 +15,9 @@
 #define BTI_J hint 36 ;
 
 /*
- * When using in-kernel BTI we need to ensure that assembly functions
- * have suitable annotations.  Override SYM_FUNC_START to insert a BTI
- * landing pad at the start of everything.
+ * When using in-kernel BTI we need to ensure that PCS-conformant assembly
+ * functions have suitable annotations.  Override SYM_FUNC_START to insert
+ * a BTI landing pad at the start of everything.
  */
 #define SYM_FUNC_START(name)				\
 	SYM_START(name, SYM_L_GLOBAL, SYM_A_ALIGN)	\
diff --git a/arch/arm64/kernel/vdso/sigreturn.S b/arch/arm64/kernel/vdso/sigreturn.S
index 3fb13b81f780..0c921130002a 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_CODE_START(__kernel_rt_sigreturn)
 	.cfi_startproc
 	.cfi_signal_frame
 	.cfi_def_cfa	x29, 0
@@ -24,6 +31,6 @@ SYM_FUNC_START(__kernel_rt_sigreturn)
 	mov	x8, #__NR_rt_sigreturn
 	svc	#0
 	.cfi_endproc
-SYM_FUNC_END(__kernel_rt_sigreturn)
+SYM_CODE_END(__kernel_rt_sigreturn)
 
 emit_aarch64_feature_1_and
diff --git a/arch/arm64/kernel/vdso32/sigreturn.S b/arch/arm64/kernel/vdso32/sigreturn.S
index 620524969696..b36d4e2267a3 100644
--- a/arch/arm64/kernel/vdso32/sigreturn.S
+++ b/arch/arm64/kernel/vdso32/sigreturn.S
@@ -17,39 +17,39 @@
 	.save {r0-r15}
 	.pad #COMPAT_SIGFRAME_REGS_OFFSET
 	nop
-SYM_FUNC_START(__kernel_sigreturn_arm)
+SYM_CODE_START(__kernel_sigreturn_arm)
 	mov r7, #__NR_compat_sigreturn
 	svc #0
 	.fnend
-SYM_FUNC_END(__kernel_sigreturn_arm)
+SYM_CODE_END(__kernel_sigreturn_arm)
 
 	.fnstart
 	.save {r0-r15}
 	.pad #COMPAT_RT_SIGFRAME_REGS_OFFSET
 	nop
-SYM_FUNC_START(__kernel_rt_sigreturn_arm)
+SYM_CODE_START(__kernel_rt_sigreturn_arm)
 	mov r7, #__NR_compat_rt_sigreturn
 	svc #0
 	.fnend
-SYM_FUNC_END(__kernel_rt_sigreturn_arm)
+SYM_CODE_END(__kernel_rt_sigreturn_arm)
 
 	.thumb
 	.fnstart
 	.save {r0-r15}
 	.pad #COMPAT_SIGFRAME_REGS_OFFSET
 	nop
-SYM_FUNC_START(__kernel_sigreturn_thumb)
+SYM_CODE_START(__kernel_sigreturn_thumb)
 	mov r7, #__NR_compat_sigreturn
 	svc #0
 	.fnend
-SYM_FUNC_END(__kernel_sigreturn_thumb)
+SYM_CODE_END(__kernel_sigreturn_thumb)
 
 	.fnstart
 	.save {r0-r15}
 	.pad #COMPAT_RT_SIGFRAME_REGS_OFFSET
 	nop
-SYM_FUNC_START(__kernel_rt_sigreturn_thumb)
+SYM_CODE_START(__kernel_rt_sigreturn_thumb)
 	mov r7, #__NR_compat_rt_sigreturn
 	svc #0
 	.fnend
-SYM_FUNC_END(__kernel_rt_sigreturn_thumb)
+SYM_CODE_END(__kernel_rt_sigreturn_thumb)
-- 
2.26.2.761.g0e0b3e54be-goog

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