[PATCH 1/2] erofs-utils: fix undefined behavior shift in erofs_init_devices

[PATCH 1/2] erofs-utils: fix undefined behavior shift in erofs_init_devices

[Nithurshen]

In erofs_init_devices(), roundup_pow_of_two() can potentially trigger
an undefined behavior shift if the incremented 'ondisk_extradevs'
value results in an overflow or an input that leads to an
out-of-bounds shift.

Promote the argument to u64 before the increment to ensure the
rounding logic operates on a safe bit-width.

Signed-off-by: Nithurshen <nithurshen.dev@gmail.com>
---
 lib/super.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/super.c b/lib/super.c
index 088c9a0..10831a7 100644
--- a/lib/super.c
+++ b/lib/super.c
@@ -49,7 +49,7 @@ static int erofs_init_devices(struct erofs_sb_info *sbi,
 		return 0;
 
 	sbi->extra_devices = ondisk_extradevs;
-	sbi->device_id_mask = roundup_pow_of_two(ondisk_extradevs + 1) - 1;
+	sbi->device_id_mask = roundup_pow_of_two((u64)ondisk_extradevs + 1) - 1;
 	sbi->devs = calloc(ondisk_extradevs, sizeof(*sbi->devs));
 	if (!sbi->devs)
 		return -ENOMEM;
-- 
2.52.0

[PATCH 2/2] erofs-utils: libzstd: fix undefined behavior shift in setdictsize

[Nithurshen]

In erofs_compressor_libzstd_setdictsize(), if pclustersize_max is 0,
dict_size becomes 0, leading to undefined behavior when calling
ilog2(0). This results in an invalid bit shift (e.g., shifting
a 32-bit value by 63 bits), as reported by cppcheck.

Fix this by adding guards to ensure dict_size is non-zero before
performing power-of-two rounding and validation.

Signed-off-by: Nithurshen <nithurshen.dev@gmail.com>
---
 lib/compressor_libzstd.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/lib/compressor_libzstd.c b/lib/compressor_libzstd.c
index 6330f44..eb768de 100644
--- a/lib/compressor_libzstd.c
+++ b/lib/compressor_libzstd.c
@@ -123,10 +123,11 @@ static int erofs_compressor_libzstd_setdictsize(struct erofs_compress *c,
 		} else {
 			dict_size = min_t(u32, Z_EROFS_ZSTD_MAX_DICT_SIZE,
 					  pclustersize_max << 3);
-			dict_size = 1U << ilog2(dict_size);
+			if (dict_size)
+				dict_size = 1U << ilog2(dict_size);
 		}
 	}
-	if (dict_size != 1U << ilog2(dict_size) ||
+	if (!dict_size || dict_size != 1U << ilog2(dict_size) ||
 	    dict_size > Z_EROFS_ZSTD_MAX_DICT_SIZE) {
 		erofs_err("invalid dictionary size %u", dict_size);
 		return -EINVAL;
-- 
2.52.0

Re: [PATCH 2/2] erofs-utils: libzstd: fix undefined behavior shift in setdictsize

[Gao Xiang]

On 2026/4/19 21:16, Nithurshen wrote:
> In erofs_compressor_libzstd_setdictsize(), if pclustersize_max is 0,

How pclustersize_max is 0? explain please?

Re: [PATCH 1/2] erofs-utils: fix undefined behavior shift in erofs_init_devices

[Gao Xiang]

On 2026/4/19 21:16, Nithurshen wrote:
> In erofs_init_devices(), roundup_pow_of_two() can potentially trigger
> an undefined behavior shift if the incremented 'ondisk_extradevs'
> value results in an overflow or an input that leads to an
> out-of-bounds shift.

I wonder why there is "out-of-bounds shift"? why you all
think it's an issue? Can you please explain in details?

ondisk_extradevs is 65535 at most.

Thanks,
Gao Xiang


> 
> Promote the argument to u64 before the increment to ensure the
> rounding logic operates on a safe bit-width.
> 
> Signed-off-by: Nithurshen <nithurshen.dev@gmail.com>
> ---
>   lib/super.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/lib/super.c b/lib/super.c
> index 088c9a0..10831a7 100644
> --- a/lib/super.c
> +++ b/lib/super.c
> @@ -49,7 +49,7 @@ static int erofs_init_devices(struct erofs_sb_info *sbi,
>   		return 0;
>   
>   	sbi->extra_devices = ondisk_extradevs;
> -	sbi->device_id_mask = roundup_pow_of_two(ondisk_extradevs + 1) - 1;
> +	sbi->device_id_mask = roundup_pow_of_two((u64)ondisk_extradevs + 1) - 1;
>   	sbi->devs = calloc(ondisk_extradevs, sizeof(*sbi->devs));
>   	if (!sbi->devs)
>   		return -ENOMEM;

[PATCH 2/2] erofs-utils: libzstd: fix undefined behavior shift in setdictsize

[Nithurshen]

Hi Xiang,

I do agree that `pclustersize_max` is always greater than 0 and
`ondisk_extradevs` is at max 65535.

My intent was to look at these functions as independent blocks.
Even though the current upstream callers provide safe inputs, I
wanted to make them robust against any undefined behavior in case
they are reused in the future.

Thanks,
Nithurshen

Regards,
Nithurshen

Re: [PATCH 2/2] erofs-utils: libzstd: fix undefined behavior shift in setdictsize

[Gao Xiang]

On 2026/4/20 10:50, Nithurshen wrote:
> Hi Xiang,
> 
> I do agree that `pclustersize_max` is always greater than 0 and
> `ondisk_extradevs` is at max 65535.
> 
> My intent was to look at these functions as independent blocks.
> Even though the current upstream callers provide safe inputs, I
> wanted to make them robust against any undefined behavior in case
> they are reused in the future.

They won't be reused in any case like this.

> 
> Thanks,
> Nithurshen
> 
> Regards,
> Nithurshen