linux-kernel - Re: [PATCH 3/3] tools/nolibc: x86_64: wrap asm functions in functions

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Message-ID: <ZrdZKcQ1SClUHWa1@biznet-home.integral.gnuweeb.org>
Date: Sat, 10 Aug 2024 19:12:25 +0700
From: Ammar Faizi <ammarfaizi2@...weeb.org>
To: Thomas Weißschuh <linux@...ssschuh.net>
Cc: Willy Tarreau <w@....eu>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] tools/nolibc: x86_64: wrap asm functions in functions

On Sat, Aug 10, 2024 at 12:54:46PM +0200, Thomas Weißschuh wrote:
> +__attribute__((weak,unused,section(".text.nolibc_memmove")))
> +__nolibc_naked __no_stack_protector
> +void *memmove(void *dst __attribute__((unused)),
> +	      const void *src __attribute__((unused)),
> +	      size_t len __attribute__((unused)))
> +{
> +	__asm__ volatile (
> +		"movq %rdx, %rcx\n\t"
> +		"movq %rdi, %rax\n\t"
> +		"movq %rdi, %rdx\n\t"
> +		"subq %rsi, %rdx\n\t"
> +		"cmpq %rcx, %rdx\n\t"
> +		"jb   .Lbackward_copy\n\t"
> +		"rep movsb\n\t"
> +		"retq\n"
> +		".Lbackward_copy:"
> +		"leaq -1(%rdi, %rcx, 1), %rdi\n\t"
> +		"leaq -1(%rsi, %rcx, 1), %rsi\n\t"
> +		"std\n\t"
> +		"rep movsb\n\t"
> +		"cld\n\t"
> +		"retq\n"
> +	);
> +	__nolibc_naked_epilogue();
> +}

NAK for this patch.

This approach appears highly dangerous, particularly when the compiler
inlines the call. When using inline assembly within a function, it's
crucial to define proper constraints and a clobber list to ensure the
arguments are correctly bound to the inline assembly.

Moreover, as it stands, there is no indication to the compiler that the
inline assembly modifies registers such as %rax, %rdi, %rsi, %rdx, %rcx,
or "memory", which could lead to unpredictable behavior.

Unfortunately, I can't spend more time on this right now as I'm
currently traveling. I'll get back to it later when I'm in transit.

-- 
Ammar Faizi