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

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

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@...ux.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