Re: [PATCH v10 05/16] arm64: ptrace: Move rseq_syscall() before audit_syscall_exit()

[Will Deacon <will@kernel.org>]

On Mon, Dec 22, 2025 at 07:47:26PM +0800, Jinjie Ruan wrote:
> commit a9f3a74a29af ("entry: Provide generic syscall exit function")
> introduce generic syscall exit function and call rseq_syscall()
> before audit_syscall_exit() and arch_syscall_exit_tracehook().
> 
> And commit b74406f37737 ("arm: Add syscall detection for restartable
> sequences") add rseq support for arm32, which also call rseq_syscall()
> before audit_syscall_exit() and tracehook_report_syscall().
> 
> However, commit 409d5db49867c ("arm64: rseq: Implement backend rseq
> calls and select HAVE_RSEQ") implement arm64 rseq and call
> rseq_syscall() after audit_syscall_exit() and tracehook_report_syscall().
> 
> So compared to the generic entry and arm32 code, arm64 terminates
> the process a bit later if the syscall is issued within
> a restartable sequence.

Given that signals are processed until later, is this actually true?

> But as commit b74406f37737 ("arm: Add syscall detection for restartable
> sequences") said, syscalls are not allowed inside restartable sequences,
> so should call rseq_syscall() at the very beginning of system call
> exiting path for CONFIG_DEBUG_RSEQ=y kernel. This could help us to detect
> whether there is a syscall issued inside restartable sequences.
> 
> It makes sense to raise SIGSEGV via rseq_syscall() before auditing
> and ptrace syscall exit, because this guarantees that the process is
> already in an error state with SIGSEGV pending when those later steps
> run. Although it makes no practical difference to signal delivery (signals
> are processed at the very end in arm64_exit_to_user_mode()), the ordering
> is more logical: detect and flag the error first, then proceed with
> the remaining work.
> 
> To make it more reasonable and in preparation for moving arm64 over to
> the generic entry code, move rseq_syscall() ahead before
> audit_syscall_exit().

I've been struggling a bit to see how this helps to align with the
generic code. I'm also concerned that rseq_debug_update_user_cs()
operates on instruction_pointer(regs) which is something that can be
chaned by ptrace.

So, I'm not saying this is wrong, but it feels like a user-visible
change that needs better justification.

Will