Re: [PATCH] block: replace deprecated strncpy with strscpy

[Justin Stitt]

I have a feeling I may have botched the subject line for this patch.
Can anyone confirm if it's good or not?

Automated tooling told me that this was the most common patch
prefix but it may be caused by large patch series that just
happened to touch this file once.

Should the subject be: nvdimm/btt: ... ?

Judging from [1] I see a few "block" and a few of nvdimm/btt.

On Wed, Oct 18, 2023 at 3:35 PM Justin Stitt <justinstitt@xxxxxxxxxx> wrote:
>
> strncpy() is deprecated for use on NUL-terminated destination strings
> [1] and as such we should prefer more robust and less ambiguous string
> interfaces.
>
> We expect super->signature to be NUL-terminated based on its usage with
> memcpy against a NUL-term'd buffer:
> btt_devs.c:
> 253 | if (memcmp(super->signature, BTT_SIG, BTT_SIG_LEN) != 0)
> btt.h:
> 13  | #define BTT_SIG "BTT_ARENA_INFO\0"
>
> NUL-padding is not required as `super` is already zero-allocated:
> btt.c:
> 985 | super = kzalloc(sizeof(struct btt_sb), GFP_NOIO);
> ... rendering any additional NUL-padding superfluous.
>
> Considering the above, a suitable replacement is `strscpy` [2] due to
> the fact that it guarantees NUL-termination on the destination buffer
> without unnecessarily NUL-padding.
>
> Let's also use the more idiomatic strscpy usage of (dest, src,
> sizeof(dest)) instead of (dest, src, XYZ_LEN) for buffers that the
> compiler can determine the size of. This more tightly correlates the
> destination buffer to the amount of bytes copied.
>
> Side note, this pattern of memcmp() on two NUL-terminated strings should
> really be changed to just a strncmp(), if i'm not mistaken? I see
> multiple instances of this pattern in this system.
>
> Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
> Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html [2]
> Link: https://github.com/KSPP/linux/issues/90
> Cc: linux-hardening@xxxxxxxxxxxxxxx
> Signed-off-by: Justin Stitt <justinstitt@xxxxxxxxxx>
> ---
> Note: build-tested only.
>
> Found with: $ rg "strncpy\("
> ---
>  drivers/nvdimm/btt.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/nvdimm/btt.c b/drivers/nvdimm/btt.c
> index d5593b0dc700..9372c36e8f76 100644
> --- a/drivers/nvdimm/btt.c
> +++ b/drivers/nvdimm/btt.c
> @@ -986,7 +986,7 @@ static int btt_arena_write_layout(struct arena_info *arena)
>         if (!super)
>                 return -ENOMEM;
>
> -       strncpy(super->signature, BTT_SIG, BTT_SIG_LEN);
> +       strscpy(super->signature, BTT_SIG, sizeof(super->signature));
>         export_uuid(super->uuid, nd_btt->uuid);
>         export_uuid(super->parent_uuid, parent_uuid);
>         super->flags = cpu_to_le32(arena->flags);
>
> ---
> base-commit: 58720809f52779dc0f08e53e54b014209d13eebb
> change-id: 20231018-strncpy-drivers-nvdimm-btt-c-15f93879989e
>
> Best regards,
> --
> Justin Stitt <justinstitt@xxxxxxxxxx>
>

[1]: https://lore.kernel.org/all/?q=dfn%3Adrivers%2Fnvdimm%2Fbtt.c

Thanks
Justin