Re: [RESEND PATCH v2] string: uninline memcpy_and_pad

[Kees Cook <keescook@chromium.org>]

On Tue, Nov 02, 2021 at 07:25:18AM -0700, Guenter Roeck wrote:
> When building m68k:allmodconfig, recent versions of gcc generate the
> following error if the length of UTS_RELEASE is less than 8 bytes.
> 
> In function 'memcpy_and_pad',
>     inlined from 'nvmet_execute_disc_identify' at
>     	drivers/nvme/target/discovery.c:268:2:
> arch/m68k/include/asm/string.h:72:25: error:
> 	'__builtin_memcpy' reading 8 bytes from a region of size 7

What things are size 8 and 7? "5.15.0\0" is 7 bytes -- is strlen()
returning _8_?? It should return 6 here.

> Discussions around the problem suggest that this only happens if an
> architecture does not provide strlen(), if -ffreestanding is provided as
> compiler option, and if CONFIG_FORTIFY_SOURCE=n. All of this is the case
> for m68k. The exact reasons are unknown, but seem to be related to the
> ability of the compiler to evaluate the return value of strlen() and
> the resulting execution flow in memcpy_and_pad(). It would be possible
> to work around the problem by using sizeof(UTS_RELEASE) instead of
> strlen(UTS_RELEASE), but that would only postpone the problem until the
> function is called in a similar way. Uninline memcpy_and_pad() instead
> to solve the problem for good.

Ew, no, this doesn't solve the problem -- it just makes the buffer sizes
invisible to the compiler. This will hide legitimate problems. Please
leave this inlined.

struct nvme_id_ctrl {
	...
        char                    fr[8];
...

        memcpy_and_pad(id->fr, sizeof(id->fr),
                       UTS_RELEASE, strlen(UTS_RELEASE), ' ');

	memcpy_and_pad(id->fr, 8, UTS_RELEASE, 6, ' ')


static inline void memcpy_and_pad(void *dest, size_t dest_len,
				  const void *src, size_t count, int pad)
	if (dest_len > count) {
		memcpy(dest, src, count);
		memset(dest + count, pad,  dest_len - count);
	} else {
		memcpy(dest, src, dest_len);
	}

->

	if (8 > 6) {
		memcpy(id->fr, UTS_RELEASE, 6);
		memset(fd->fr + 6, ' ', 2);
	}

Ah, I've found the earlier thread now:
https://lore.kernel.org/all/CAMuHMdX365qmWiii=gQLADpW49EMkdDrVJDPWNBpAZuZM0WQFQ@mail.gmail.com/

This looks like a compiler bug in that it isn't collapsing the strlen()
into a compile-time constant. "sizeof(UTS_RELEASE) - 1" seems a
reasonable work-around?

I'd suggest reporting this to GCC. I see two bugs:

- strlen() not getting replace with a constant expression
- warning being generated across a runtime check

it seems like GCC *thinks* it knows strlen will be a CE, but then
instead it keeps both branches anyway, triggering a warning.

-Kees

> 
> Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Geert Uytterhoeven <geert@linux-m68k.org>
> Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
> Acked-by: Andy Shevchenko <andriy.shevchenko@intel.com>
> Signed-off-by: Guenter Roeck <linux@roeck-us.net>
> ---
> v2: Moved to lib/string_helpers.c
>     Balanced { } in if/else statement to make checkpatch happy
>     Added Reviewed-by: /Acked-by: tags
> 
>  include/linux/string.h | 19 ++-----------------
>  lib/string_helpers.c   | 20 ++++++++++++++++++++
>  2 files changed, 22 insertions(+), 17 deletions(-)
> 
> diff --git a/include/linux/string.h b/include/linux/string.h
> index 5a36608144a9..b6572aeca2f5 100644
> --- a/include/linux/string.h
> +++ b/include/linux/string.h
> @@ -253,23 +253,8 @@ static inline const char *kbasename(const char *path)
>  #include <linux/fortify-string.h>
>  #endif
>  
> -/**
> - * memcpy_and_pad - Copy one buffer to another with padding
> - * @dest: Where to copy to
> - * @dest_len: The destination buffer size
> - * @src: Where to copy from
> - * @count: The number of bytes to copy
> - * @pad: Character to use for padding if space is left in destination.
> - */
> -static inline void memcpy_and_pad(void *dest, size_t dest_len,
> -				  const void *src, size_t count, int pad)
> -{
> -	if (dest_len > count) {
> -		memcpy(dest, src, count);
> -		memset(dest + count, pad,  dest_len - count);
> -	} else
> -		memcpy(dest, src, dest_len);
> -}
> +void memcpy_and_pad(void *dest, size_t dest_len, const void *src, size_t count,
> +		    int pad);
>  
>  /**
>   * memset_after - Set a value after a struct member to the end of a struct
> diff --git a/lib/string_helpers.c b/lib/string_helpers.c
> index faa9d8e4e2c5..d5d008f5b1d9 100644
> --- a/lib/string_helpers.c
> +++ b/lib/string_helpers.c
> @@ -883,6 +883,26 @@ char *strreplace(char *s, char old, char new)
>  }
>  EXPORT_SYMBOL(strreplace);
>  
> +/**
> + * memcpy_and_pad - Copy one buffer to another with padding
> + * @dest: Where to copy to
> + * @dest_len: The destination buffer size
> + * @src: Where to copy from
> + * @count: The number of bytes to copy
> + * @pad: Character to use for padding if space is left in destination.
> + */
> +void memcpy_and_pad(void *dest, size_t dest_len, const void *src, size_t count,
> +		    int pad)
> +{
> +	if (dest_len > count) {
> +		memcpy(dest, src, count);
> +		memset(dest + count, pad,  dest_len - count);
> +	} else {
> +		memcpy(dest, src, dest_len);
> +	}
> +}
> +EXPORT_SYMBOL(memcpy_and_pad);
> +
>  #ifdef CONFIG_FORTIFY_SOURCE
>  void fortify_panic(const char *name)
>  {
> -- 
> 2.33.0
> 

-- 
Kees Cook