Re: [PATCH v2 1/1] arm64: Do not forget syscall when starting a new thread.

From: Will Deacon <will@kernel.org>
To: Francis Laniel <flaniel@linux.microsoft.com>
Cc: linux-trace-devel@vger.kernel.org,
	James Morse <james.morse@arm.com>,
	Daniel Kiss <daniel.kiss@arm.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Mark Brown <broonie@kernel.org>,
	Peter Collingbourne <pcc@google.com>,
	Kees Cook <keescook@chromium.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Christophe Leroy <christophe.leroy@csgroup.eu>,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 1/1] arm64: Do not forget syscall when starting a new thread.
Date: Tue, 28 Jun 2022 14:58:35 +0100	[thread overview]
Message-ID: <20220628135834.GA24116@willie-the-truck> (raw)
In-Reply-To: <20220608162447.666494-2-flaniel@linux.microsoft.com>

On Wed, Jun 08, 2022 at 05:24:46PM +0100, Francis Laniel wrote:
> This patch enables exeve*() to be traced with syscalls:sys_exit_execve
> tracepoint.
> Previous to it, by calling forget_syscall(), this tracepoint would not
> print its information as syscall is -1.
> So, this patch removes call to forget_syscall() and set regs->syscallno
> to its previous value.
> 
> Signed-off-by: Francis Laniel <flaniel@linux.microsoft.com>
> ---
>  arch/arm64/include/asm/processor.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/include/asm/processor.h b/arch/arm64/include/asm/processor.h
> index 9e58749db21d..86eb0bfe3b38 100644
> --- a/arch/arm64/include/asm/processor.h
> +++ b/arch/arm64/include/asm/processor.h
> @@ -272,8 +272,9 @@ void tls_preserve_current_state(void);
>  
>  static inline void start_thread_common(struct pt_regs *regs, unsigned long pc)
>  {
> +	s32 previous_syscall = regs->syscallno;
>  	memset(regs, 0, sizeof(*regs));
> -	forget_syscall(regs);
> +	regs->syscallno = previous_syscall;

I'm still unsure about this. Even if we preserve the syscall number here,
won't all the arguments be reported as 0?

I also looked quickly at the 32-bit arch/arm/ code and it looks like the
same behaviour exists there (module CONFIG_BINFMT_ELF_FDPIC).

Will