Re: [PATCH] Fix regression in memrchr

CC the author of c9b74e328, Alexey Lapshin.

Alexey, any input?


Thanks,
Corinna




On Mar 19 08:55, Federico Terraneo wrote:
> Hi,
> while upgrading our toolchain for the Miosix kernel to use the lastest
> Newlib release, we noticed a regression in memrchr. This issue causes an
> unaligned memory access resulting in a fault in ARM Cortex M0
> microcontrollers.
> 
> The bug has been introduced in commit
> c9b74e32892966c8f66280c752181292bc95f690, where the macro UNALIGNED was
> replaced with a generic one UNALIGNED_X.
> However, in memrchr.c the UNALIGNED macro was defined as
> 
> #define UNALIGNED(X) ((long)(X + 1) & (sizeof (long) - 1))
> 
> thus checking if X+1 is aligned, not if X is aligned.
> 
> The replacement macro checks if X is aligned, thus breaking the algorithm.
> 
> The issue was found by my colleague Daniele Cattaneo.
> 
> Best regards,
> Federico Terraneo

> From ea9b7d9f09d48b2bf88682d47992f5ee0bd4e57d Mon Sep 17 00:00:00 2001
> From: Daniele Cattaneo <[email protected]>
> Date: Wed, 18 Mar 2026 17:33:02 +0100
> Subject: [PATCH] libc: string: Fix off-by-one alignment bug in memrchr.
> 
> The loop at line 50 should test the bytes in the buffer, from the end towards
> the beginning, until it reaches a character whose address is aligned, leaving
> `src' pointing to the next character to test. However, the loop condition did
> not test the address of the last character read (`src + 1'), but the address of
> the next character (`src'). As a consequence, a misaligned address was computed
> at line 69, because the (aligned) address in `src' is incremented by 1.
> 
> This bug was originally introduced in commit c9b74e328 during a refactoring of
> the macros used by string functions. Before the refactoring, the +1 increment
> in the loop condition was located in the macros specific to memrchr.
> ---
>  newlib/libc/string/memrchr.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/newlib/libc/string/memrchr.c b/newlib/libc/string/memrchr.c
> index 0a0c80fd9..b56e2b476 100644
> --- a/newlib/libc/string/memrchr.c
> +++ b/newlib/libc/string/memrchr.c
> @@ -47,7 +47,7 @@ memrchr (const void *src_void,
>    unsigned long  mask;
>    unsigned int i;
>  
> -  while (UNALIGNED_X(src))
> +  while (UNALIGNED_X(src + 1))
>      {
>        if (!length--)
>          return NULL;
> -- 
> 2.53.0
>