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 2/3] arm64: vdso: Add a comment to justify the mysterious NOP in sigreturn
Date: Tue, 19 May 2020 13:18:17 +0100 [thread overview]
Message-ID: <20200519121818.14511-3-will@kernel.org> (raw)
In-Reply-To: <20200519121818.14511-1-will@kernel.org>
Every so often we have to remind ourselves about the purpose of the
weird NOP instruction immediately preceding the sigreturn trampoline.
Add a short comment to state that it exists for some unwinders that
determine the caller address by subtracting from the return address.
Cc: Dave Martin <dave.martin@arm.com>
Cc: Daniel Kiss <daniel.kiss@arm.com>
Cc: Tamas Zsoldos <tamas.zsoldos@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>
---
arch/arm64/kernel/vdso/sigreturn.S | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)
diff --git a/arch/arm64/kernel/vdso/sigreturn.S b/arch/arm64/kernel/vdso/sigreturn.S
index 83ac284dae79..7853fa9692f6 100644
--- a/arch/arm64/kernel/vdso/sigreturn.S
+++ b/arch/arm64/kernel/vdso/sigreturn.S
@@ -14,7 +14,12 @@
.text
- nop
+/*
+ * This mysterious NOP is required for some unwinders that subtract one from
+ * the return address in order to identify the calling function.
+ * Hack borrowed from arch/powerpc/kernel/vdso64/sigtramp.S.
+ */
+ nop // Mysterious NOP
/*
* GDB relies on being able to identify the sigreturn instruction sequence to
* unwind from signal handlers. We cannot, therefore, use SYM_FUNC_START()
--
2.26.2.761.g0e0b3e54be-goog
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next prev parent reply other threads:[~2020-05-19 12:19 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 ` [PATCH 1/3] arm64: vdso: Don't prefix sigreturn trampoline with a BTI C instruction Will Deacon
2020-05-19 12:38 ` 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 ` Will Deacon [this message]
2020-05-19 13:26 ` [PATCH 2/3] arm64: vdso: Add a comment to justify the mysterious NOP in sigreturn 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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