linux-kernel - Re: [PATCH] ELF: use __builtin_mul

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Message-Id: <20230421123911.3c4b1e3b56781ffdf043ef58@linux-foundation.org>
Date:   Fri, 21 Apr 2023 12:39:11 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Alexey Dobriyan <adobriyan@...il.com>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ELF: use __builtin_mul_overflow() more

On Fri, 21 Apr 2023 21:54:36 +0300 Alexey Dobriyan <adobriyan@...il.com> wrote:

> __builtin_mul_overflow() can do multiplication and overflow check
> in one line.
> 
> --- a/fs/binfmt_elf.c
> +++ b/fs/binfmt_elf.c
> @@ -1651,9 +1651,8 @@ static int fill_files_note(struct memelfnote *note, struct coredump_params *cprm
>  
>  	/* *Estimated* file count and total data size needed */
>  	count = cprm->vma_count;
> -	if (count > UINT_MAX / 64)
> +	if (__builtin_mul_overflow(count, 64, &size))
>  		return -EINVAL;
> -	size = count * 64;

Huh, what the heck is that ;)


include/linux/overflow.h has check_mul_overflow() for us to use here.


tools/lib/bpf/libbpf_internal.h uses

	#if __has_builtin(__builtin_mul_overflow)

but check_mul_overflow() didn't bother testing for availability. 
Probably tools/lib/bpf/libbpf_internal.h should just use
check_mul_overflow().