linux-kernel - Re: [RFC PATCH v1 1/1] arm64: Forget syscall if different from execve*()

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Message-ID: <20220510105948.GB27557@willie-the-truck>
Date:   Tue, 10 May 2022 11:59:48 +0100
From:   Will Deacon <will@...nel.org>
To:     Francis Laniel <flaniel@...ux.microsoft.com>
Cc:     linux-arm-kernel@...ts.infradead.org,
        linux-trace-devel@...r.kernel.org,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Brown <broonie@...nel.org>,
        Peter Collingbourne <pcc@...gle.com>,
        Mark Rutland <mark.rutland@....com>,
        Kees Cook <keescook@...omium.org>,
        Daniel Kiss <daniel.kiss@....com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v1 1/1] arm64: Forget syscall if different from
 execve*()

On Mon, May 09, 2022 at 04:19:57PM +0100, Francis Laniel wrote:
> This patch enables exeve*() to be traced by syscalls:sys_exit_execve
> tracepoint.
> Previously, calling forget_syscall() would set syscall to -1, which impedes
> this tracepoint to prints its information.
> So, this patch makes call to forget_syscall() conditional by only calling
> it when syscall number is not execve() or execveat().
> 
> Signed-off-by: Francis Laniel <flaniel@...ux.microsoft.com>
> ---
>  arch/arm64/include/asm/processor.h | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/include/asm/processor.h b/arch/arm64/include/asm/processor.h
> index 73e38d9a540c..e12ceb363d6a 100644
> --- a/arch/arm64/include/asm/processor.h
> +++ b/arch/arm64/include/asm/processor.h
> @@ -34,6 +34,8 @@
>  
>  #include <vdso/processor.h>
>  
> +#include <asm-generic/unistd.h>
> +
>  #include <asm/alternative.h>
>  #include <asm/cpufeature.h>
>  #include <asm/hw_breakpoint.h>
> @@ -250,8 +252,12 @@ 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);
> +	if (previous_syscall == __NR_execve || previous_syscall == __NR_execveat)
> +		regs->syscallno = previous_syscall;
> +	else
> +		forget_syscall(regs);

Hmm, this really looks like a bodge and it doesn't handle the compat case
either.

How do other architectures handle this?

Will