From: Will Deacon <will@kernel.org>
To: linux-arm-kernel@lists.infradead.org
Cc: Will Deacon <will@kernel.org>,
Tamas Zsoldos <tamas.zsoldos@arm.com>,
Mark Brown <broonie@kernel.org>,
kernel-team@android.com, Dave Martin <dave.martin@arm.com>,
Daniel Kiss <daniel.kiss@arm.com>
Subject: [PATCH 1/3] arm64: vdso: Don't prefix sigreturn trampoline with a BTI C instruction
Date: Tue, 19 May 2020 13:18:16 +0100 [thread overview]
Message-ID: <20200519121818.14511-2-will@kernel.org> (raw)
In-Reply-To: <20200519121818.14511-1-will@kernel.org>
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 <dave.martin@arm.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Daniel Kiss <daniel.kiss@arm.com>
Cc: Tamas Zsoldos <tamas.zsoldos@arm.com>
Fixes: 714a8d02ca4d ("arm64: asm: Override SYM_FUNC_START when building the kernel with BTI")
Signed-off-by: Will Deacon <will@kernel.org>
---
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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next prev parent reply other threads:[~2020-05-19 12:18 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-19 12:18 [PATCH 0/3] arm64 sigreturn unwinding fixes Will Deacon
2020-05-19 12:18 ` Will Deacon [this message]
2020-05-19 12:38 ` [PATCH 1/3] arm64: vdso: Don't prefix sigreturn trampoline with a BTI C instruction Mark Brown
2020-05-19 13:25 ` Dave Martin
2020-05-19 14:35 ` Mark Brown
2020-05-19 14:55 ` Dave Martin
2020-05-19 15:42 ` Mark Brown
2020-05-20 9:48 ` Dave Martin
2020-05-20 10:46 ` Mark Brown
2020-05-20 11:08 ` Dave Martin
2020-05-19 15:26 ` Will Deacon
2020-05-19 13:21 ` Dave Martin
2020-05-19 13:29 ` Will Deacon
2020-05-19 12:18 ` [PATCH 2/3] arm64: vdso: Add a comment to justify the mysterious NOP in sigreturn Will Deacon
2020-05-19 13:26 ` Dave Martin
2020-05-19 12:18 ` [PATCH 3/3] arm64: vdso: Fix CFI directives in sigreturn trampoline Will Deacon
2020-05-19 13:09 ` Dave P Martin
2020-05-19 13:39 ` Will Deacon
2020-05-19 13:55 ` Dave Martin
2020-05-19 15:24 ` Will Deacon
2020-05-19 15:30 ` Daniel Kiss
2020-05-19 15:55 ` Will Deacon
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