Subject: Re: [PATCH v3 1/1] fs/proc: Fix build with GCC 15 due to -Werror=unterminated-string-initialization

From: Andrew Morton <[email-masked]>
Date: Mon, 7 Oct 2024 19:08:13 -0700

On Sat,  5 Oct 2024 12:07:00 +0530 Brahmajit Das <[email-masked]> wrote:

> show show_smap_vma_flags() has been using misspelled initializer in
> mnemonics[] - it needed to initialize 2 element array of char and it
> used NUL-padded 2 character string literals (i.e. 3-element
> initializer).
> 
> This has been spotted by gcc-15[*]; prior to that gcc quietly dropped
> the 3rd eleemnt of initializers. To fix this we are increasing the size
> of mnemonics[] (from mnemonics[BITS_PER_LONG][2] to
> mnemonics[BITS_PER_LONG][3]) to accomodate the NUL-padded string
> literals.
> 
> This also helps us in simplyfying the logic for printing of the flags as
> instead of printing each character from the mnemonics[], we can just
> print the mnemonics[] using seq_printf.
> 
> [*]: fs/proc/task_mmu.c:917:49: error: initializer-string for array of ‘char’ is too long [-Werror=unterminate d-string-initialization]
>   917 |                 [0 ... (BITS_PER_LONG-1)] = "??",
>       |                                                 ^~~~
> fs/proc/task_mmu.c:917:49: error: initializer-string for array of ‘char’ is too long [-Werror=unterminate d-string-initialization]
> fs/proc/task_mmu.c:917:49: error: initializer-string for array of ‘char’ is too long [-Werror=unterminate d-string-initialization]
> fs/proc/task_mmu.c:917:49: error: initializer-string for array of ‘char’ is too long [-Werror=unterminate d-string-initialization]
> fs/proc/task_mmu.c:917:49: error: initializer-string for array of ‘char’ is too long [-Werror=unterminate d-string-initialization]
> fs/proc/task_mmu.c:917:49: error: initializer-string for array of ‘char’ is too long [-Werror=unterminate d-string-initialization]

> Cc: [email-masked], [email-masked], [email-masked], [email-masked]

It is strange to cc only linux-next.  It isn't really a development
mailing list.  Please include an appropriate development list and/or
linux-kernel on patches.

> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -909,8 +909,15 @@ static void show_smap_vma_flags(struct seq_file *m, struct vm_area_struct *vma)
>  {
>  	/*
>  	 * Don't forget to update Documentation/ on changes.
> +	 *
> +	 * The length of the second argument of mnemonics[]
> +	 * needs to be 3 instead of previously set 2
> +	 * (i.e. from [BITS_PER_LONG][2] to [BITS_PER_LONG][3])
> +	 * to avoid spurious
> +	 * -Werror=unterminated-string-initialization warning
> +	 *  with GCC 15
>  	 */
> -	static const char mnemonics[BITS_PER_LONG][2] = {
> +	static const char mnemonics[BITS_PER_LONG][3] = {
>  		/*
>  		 * In case if we meet a flag we don't know about.
>  		 */
> @@ -987,11 +994,8 @@ static void show_smap_vma_flags(struct seq_file *m, struct vm_area_struct *vma)
>  	for (i = 0; i < BITS_PER_LONG; i++) {
>  		if (!mnemonics[i][0])
>  			continue;
> -		if (vma->vm_flags & (1UL << i)) {
> -			seq_putc(m, mnemonics[i][0]);
> -			seq_putc(m, mnemonics[i][1]);
> -			seq_putc(m, ' ');
> -		}
> +		if (vma->vm_flags & (1UL << i))
> +			seq_printf(m, "%s ", mnemonics[i]);
>  	}
>  	seq_putc(m, '\n');
>  }

Patch looks good.  I'll add this to the changelog:

Stephen pointed out:

: The C standard explicitly allows for a string initializer to be too long
: due to the NUL byte at the end ...  so this warning may be overzealous.

but let's make the warning go away anwyay.