Re: [PATCH] fat: avoid stack overflow warning

[David Laight <david.laight.linux@gmail.com>]

On Fri, 15 May 2026 22:44:46 +0200
Arnd Bergmann <arnd@kernel.org> wrote:

> From: Arnd Bergmann <arnd@arndb.de>
> 
> Building the fat kunit tests on with -fsanitize=alignment
> reveals some rather excessive stack usage:
> 
> fs/fat/fat_test.c: In function 'fat_clus_to_blknr_test':
> fs/fat/fat_test.c:33:1: error: the frame size of 4736 bytes is larger than 1536 bytes [-Werror=frame-larger-than=]
>    33 | }
>       | ^
> fs/fat/fat_test.c: In function 'fat_get_blknr_offset_test':
> fs/fat/fat_test.c:52:1: error: the frame size of 4800 bytes is larger than 1536 bytes [-Werror=frame-larger-than=]
> 
> The problem is clearly related to the on-stack copy of a local
> msdos_sb_info structure. Avoid this by making that copy
> 'static const' and changing the called functions to accept
> a constant input.

Just FYI:

Without -fsanitize=alignment fat_clus_to_blknr() is almost certainly
inlined (as expected) and then 'sbi' is completely optimised away.

-- David

> 
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
>  fs/fat/fat.h      | 4 ++--
>  fs/fat/fat_test.c | 4 ++--
>  2 files changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/fat/fat.h b/fs/fat/fat.h
> index 5a58f0bf8ce8..52bced59abe4 100644
> --- a/fs/fat/fat.h
> +++ b/fs/fat/fat.h
> @@ -247,13 +247,13 @@ static inline unsigned char fat_checksum(const __u8 *name)
>  	return s;
>  }
>  
> -static inline sector_t fat_clus_to_blknr(struct msdos_sb_info *sbi, int clus)
> +static inline sector_t fat_clus_to_blknr(const struct msdos_sb_info *sbi, int clus)
>  {
>  	return ((sector_t)clus - FAT_START_ENT) * sbi->sec_per_clus
>  		+ sbi->data_start;
>  }
>  
> -static inline void fat_get_blknr_offset(struct msdos_sb_info *sbi,
> +static inline void fat_get_blknr_offset(const struct msdos_sb_info *sbi,
>  				loff_t i_pos, sector_t *blknr, int *offset)
>  {
>  	*blknr = i_pos >> sbi->dir_per_block_bits;
> diff --git a/fs/fat/fat_test.c b/fs/fat/fat_test.c
> index 4eeed9dca549..9583ce66dca3 100644
> --- a/fs/fat/fat_test.c
> +++ b/fs/fat/fat_test.c
> @@ -22,7 +22,7 @@ static void fat_checksum_test(struct kunit *test)
>  
>  static void fat_clus_to_blknr_test(struct kunit *test)
>  {
> -	struct msdos_sb_info sbi = {
> +	static const struct msdos_sb_info sbi = {
>  		.sec_per_clus = 4,
>  		.data_start = 100,
>  	};
> @@ -34,7 +34,7 @@ static void fat_clus_to_blknr_test(struct kunit *test)
>  
>  static void fat_get_blknr_offset_test(struct kunit *test)
>  {
> -	struct msdos_sb_info sbi = {
> +	static const struct msdos_sb_info sbi = {
>  		.dir_per_block = 16,
>  		.dir_per_block_bits = 4,
>  	};