Re: [PATCH v4 1/2] string: Add load_unaligned_zeropad() code path to sized_strscpy()

[Catalin Marinas <catalin.marinas@arm.com>]

On Fri, Mar 28, 2025 at 05:03:36PM -0700, Peter Collingbourne wrote:
> diff --git a/lib/string.c b/lib/string.c
> index eb4486ed40d25..b632c71df1a50 100644
> --- a/lib/string.c
> +++ b/lib/string.c
> @@ -119,6 +119,7 @@ ssize_t sized_strscpy(char *dest, const char *src, size_t count)
>  	if (count == 0 || WARN_ON_ONCE(count > INT_MAX))
>  		return -E2BIG;
>  
> +#ifndef CONFIG_DCACHE_WORD_ACCESS
>  #ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
>  	/*
>  	 * If src is unaligned, don't cross a page boundary,
> @@ -133,12 +134,14 @@ ssize_t sized_strscpy(char *dest, const char *src, size_t count)
>  	/* If src or dest is unaligned, don't do word-at-a-time. */
>  	if (((long) dest | (long) src) & (sizeof(long) - 1))
>  		max = 0;
> +#endif
>  #endif
>  
>  	/*
> -	 * read_word_at_a_time() below may read uninitialized bytes after the
> -	 * trailing zero and use them in comparisons. Disable this optimization
> -	 * under KMSAN to prevent false positive reports.
> +	 * load_unaligned_zeropad() or read_word_at_a_time() below may read
> +	 * uninitialized bytes after the trailing zero and use them in
> +	 * comparisons. Disable this optimization under KMSAN to prevent
> +	 * false positive reports.
>  	 */
>  	if (IS_ENABLED(CONFIG_KMSAN))
>  		max = 0;
> @@ -146,7 +149,11 @@ ssize_t sized_strscpy(char *dest, const char *src, size_t count)
>  	while (max >= sizeof(unsigned long)) {
>  		unsigned long c, data;
>  
> +#ifdef CONFIG_DCACHE_WORD_ACCESS
> +		c = load_unaligned_zeropad(src+res);
> +#else
>  		c = read_word_at_a_time(src+res);
> +#endif
>  		if (has_zero(c, &data, &constants)) {
>  			data = prep_zero_mask(c, data, &constants);
>  			data = create_zero_mask(data);

Kees mentioned the scenario where this crosses the page boundary and we
pad the source with zeros. It's probably fine but there are 70+ cases
where the strscpy() return value is checked, I only looked at a couple.

Could we at least preserve the behaviour with regards to page boundaries
and keep the existing 'max' limiting logic? If I read the code
correctly, a fall back to reading one byte at a time from an unmapped
page would panic. We also get this behaviour if src[0] is reading from
an invalid address, though for arm64 the panic would be in
ex_handler_load_unaligned_zeropad() when count >= 8.

Reading across tag granule (but not across page boundary) and causing a
tag check fault would result in padding but we can live with this and
only architectures that do MTE-style tag checking would get the new
behaviour.

What I haven't checked is whether a tag check fault in
ex_handler_load_unaligned_zeropad() would confuse the KASAN logic for
MTE (it would be a second tag check fault while processing the first).
At a quick look, it seems ok but it might be worth checking.

-- 
Catalin