linux-kernel - Re: [PATCH] bootconfig: Fix negative seeks on 32-bit with LFS enabled

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Message-Id: <20250820100731.df29750fa21d864d18169d5c@kernel.org>
Date: Wed, 20 Aug 2025 10:07:31 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Ben Hutchings <benh@...ian.org>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH] bootconfig: Fix negative seeks on 32-bit with LFS
 enabled

On Sun, 17 Aug 2025 16:21:46 +0200
Ben Hutchings <benh@...ian.org> wrote:

> Commit 26dda5769509 "tools/bootconfig: Cleanup bootconfig footer size
> calculations" replaced some expressions of type int with the
> BOOTCONFIG_FOOTER_SIZE macro, which expands to an expression of type
> size_t, which is unsigned.
> 
> On 32-bit architectures with LFS enabled (i.e. off_t is 64-bit), the
> seek offset of -BOOTCONFIG_FOOTER_SIZE now turns into a positive
> value.

Oops, I thought the sign bit would be extended.

> 
> Fix this by casting the size to off_t before negating it.
> 
> Just in case someone changes BOOTCONFIG_MAGIC_LEN to have type size_t
> later, do the same thing to the seek offset of -BOOTCONFIG_MAGIC_LEN.
> 
> Fixes: 26dda5769509 ("tools/bootconfig: Cleanup bootconfig footer size calculations")
> Signed-off-by: Ben Hutchings <benh@...ian.org>

Thanks for the fix!

> ---
>  tools/bootconfig/main.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/bootconfig/main.c b/tools/bootconfig/main.c
> index 57c669d2aa90..55d59ed507d5 100644
> --- a/tools/bootconfig/main.c
> +++ b/tools/bootconfig/main.c
> @@ -193,7 +193,7 @@ static int load_xbc_from_initrd(int fd, char **buf)
>  	if (stat.st_size < BOOTCONFIG_FOOTER_SIZE)
>  		return 0;
>  
> -	if (lseek(fd, -BOOTCONFIG_MAGIC_LEN, SEEK_END) < 0)
> +	if (lseek(fd, -(off_t)BOOTCONFIG_MAGIC_LEN, SEEK_END) < 0)
>  		return pr_errno("Failed to lseek for magic", -errno);
>  
>  	if (read(fd, magic, BOOTCONFIG_MAGIC_LEN) < 0)
> @@ -203,7 +203,7 @@ static int load_xbc_from_initrd(int fd, char **buf)
>  	if (memcmp(magic, BOOTCONFIG_MAGIC, BOOTCONFIG_MAGIC_LEN) != 0)
>  		return 0;
>  
> -	if (lseek(fd, -BOOTCONFIG_FOOTER_SIZE, SEEK_END) < 0)
> +	if (lseek(fd, -(off_t)BOOTCONFIG_FOOTER_SIZE, SEEK_END) < 0)
>  		return pr_errno("Failed to lseek for size", -errno);
>  
>  	if (read(fd, &size, sizeof(uint32_t)) < 0)


-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>