[PATCH] arch/riscv: vdso: remove CFI landing pad from rt_sigreturn

[Aurelien Jarno <aurelien@aurel32.net>]

When CONFIG_RISCV_USER_CFI is enabled, the CFI version of the vDSO, has
a CFI landing pad instruction at the start of __vdso_rt_sigreturn. This
breaks libgcc's unwinding code which matches on the first two
instructions. Other unwinders that rely on similar instruction matching
may also be affected.

Since __vdso_rt_sigreturn is reached as part of signal-return handling
rather than via an indirect call/jump from userspace, it does not need a
CFI landing pad. Remove it and restore the instruction sequence expected
by existing unwinding code.

This matches what was done on arm64 in commit 9a964285572b ("arm64:
vdso: Don't prefix sigreturn trampoline with a BTI C instruction") for a
similar issue.

Fixes: 37f57bd3faea ("arch/riscv: compile vdso with landing pad and shadow stack note")
Co-authored-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
---
 arch/riscv/kernel/vdso/rt_sigreturn.S | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/kernel/vdso/rt_sigreturn.S b/arch/riscv/kernel/vdso/rt_sigreturn.S
index e82987dc37394..f6b053d3bb798 100644
--- a/arch/riscv/kernel/vdso/rt_sigreturn.S
+++ b/arch/riscv/kernel/vdso/rt_sigreturn.S
@@ -7,11 +7,18 @@
 #include <asm/unistd.h>
 #include <asm/assembler.h>
 
+/* WARNING: Do NOT add a CFI landing pad at the start of this function.
+ * Unwinders such as libgcc identify the sigreturn trampoline by matching the
+ * instruction sequence. Adding a landing pad here would break unwinding from
+ * signal handlers.
+ *
+ * This trampoline is used only for signal return and not via an indirect
+ * call/jump from userspace, so adding CFI landing pad is unnecessary.
+ */
 	.text
 SYM_FUNC_START(__vdso_rt_sigreturn)
 	.cfi_startproc
 	.cfi_signal_frame
-	vdso_lpad
 	li a7, __NR_rt_sigreturn
 	ecall
 	.cfi_endproc
-- 
2.53.0