arm64: vdso: Don't prefix sigreturn trampoline with a BTI C instruction

For better or worse, GDB relies on the exact instruction sequence in the
VDSO sigreturn trampoline in order to unwind from signals correctly.
Commit c91db232 ("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 <daniel.kiss@arm.com>
Cc: Tamas Zsoldos <tamas.zsoldos@arm.com>
Reviewed-by: Dave Martin <dave.martin@arm.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Fixes: c91db232 ("arm64: vdso: Convert to modern assembler annotations")
Signed-off-by: Will Deacon <will@kernel.org>

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)	\

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

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)