From: Will Deacon <will@kernel.org>
To: linux-arm-kernel@lists.infradead.org
Cc: Mark Rutland <mark.rutland@arm.com>,
Luis Machado <luis.machado@linaro.org>,
Kees Cook <keescook@chromium.org>, Will Deacon <will@kernel.org>,
catalin.marinas@arm.com, Keno Fischer <keno@juliacomputing.com>,
kernel-team@android.com
Subject: [PATCH v3 5/7] arm64: syscall: Expand the comment about ptrace and syscall(-1)
Date: Fri, 10 Jul 2020 14:07:00 +0100 [thread overview]
Message-ID: <20200710130702.30658-6-will@kernel.org> (raw)
In-Reply-To: <20200710130702.30658-1-will@kernel.org>
If a task executes syscall(-1), we intercept this early and force x0 to
be -ENOSYS so that we don't need to distinguish this scenario from one
where the scno is -1 because a tracer wants to skip the system call
using ptrace. With the return value set, the return path is the same as
the skip case.
Although there is a one-line comment noting this in el0_svc_common(), it
misses out most of the detail. Expand the comment to describe a bit more
about what is going on.
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Keno Fischer <keno@juliacomputing.com>
Cc: Luis Machado <luis.machado@linaro.org>
Signed-off-by: Will Deacon <will@kernel.org>
---
arch/arm64/kernel/syscall.c | 16 +++++++++++++++-
1 file changed, 15 insertions(+), 1 deletion(-)
diff --git a/arch/arm64/kernel/syscall.c b/arch/arm64/kernel/syscall.c
index 98a26d4e7b0c..5f0c04863d2c 100644
--- a/arch/arm64/kernel/syscall.c
+++ b/arch/arm64/kernel/syscall.c
@@ -124,7 +124,21 @@ static void el0_svc_common(struct pt_regs *regs, int scno, int sc_nr,
user_exit();
if (has_syscall_work(flags)) {
- /* set default errno for user-issued syscall(-1) */
+ /*
+ * The de-facto standard way to skip a system call using ptrace
+ * is to set the system call to -1 (NO_SYSCALL) and set x0 to a
+ * suitable error code for consumption by userspace. However,
+ * this cannot be distinguished from a user-issued syscall(-1)
+ * and so we must set x0 to -ENOSYS here in case the tracer doesn't
+ * issue the skip and we fall into trace_exit with x0 preserved.
+ *
+ * This is slightly odd because it also means that if a tracer
+ * sets the system call number to -1 but does not initialise x0,
+ * then x0 will be preserved for all system calls apart from a
+ * user-issued syscall(-1). However, requesting a skip and not
+ * setting the return value is unlikely to do anything sensible
+ * anyway.
+ */
if (scno == NO_SYSCALL)
regs->regs[0] = -ENOSYS;
scno = syscall_trace_enter(regs);
--
2.27.0.383.g050319c2ae-goog
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next prev parent reply other threads:[~2020-07-10 13:08 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-10 13:06 [PATCH v3 0/7] arm64: Fix single-step handling and syscall tracing Will Deacon
2020-07-10 13:06 ` [PATCH v3 1/7] arm64: ptrace: Consistently use pseudo-singlestep exceptions Will Deacon
2020-07-16 0:27 ` Sasha Levin
2020-07-10 13:06 ` [PATCH v3 2/7] arm64: ptrace: Override SPSR.SS when single-stepping is enabled Will Deacon
2020-07-16 0:27 ` Sasha Levin
2020-07-10 13:06 ` [PATCH v3 3/7] arm64: compat: Ensure upper 32 bits of x0 are zero on syscall return Will Deacon
2020-07-16 0:27 ` Sasha Levin
2020-07-10 13:06 ` [PATCH v3 4/7] arm64: ptrace: Add a comment describing our syscall entry/exit trap ABI Will Deacon
2020-07-10 13:07 ` Will Deacon [this message]
2020-07-10 13:07 ` [PATCH v3 6/7] arm64: ptrace: Use NO_SYSCALL instead of -1 in syscall_trace_enter() Will Deacon
2020-07-10 16:04 ` Kees Cook
2020-07-10 16:11 ` Will Deacon
2020-07-13 2:32 ` Kees Cook
2020-07-10 13:07 ` [PATCH v3 7/7] arm64: Use test_tsk_thread_flag() for checking TIF_SINGLESTEP Will Deacon
2020-07-14 11:57 ` [PATCH v3 0/7] arm64: Fix single-step handling and syscall tracing Luis Machado
2020-07-15 12:25 ` Luis Machado
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