[PATCH] erofs-utils: lib: fix undefined behavior in zstd dict_size bit shift

[PATCH] erofs-utils: lib: fix undefined behavior in zstd dict_size bit shift

[Nithurshen]

cppcheck static analysis flags that shifting the signed 32-bit literal
`1` by `ilog2(dict_size)` can lead to undefined behavior if the shift
amount reaches or exceeds 31.

This patch casts the literal to `1ULL` to ensure the shift operates
safely on an unsigned 64-bit integer, preventing potential overflows
on different architectures.

Signed-off-by: Nithurshen <nithurshen@gmail.com>
---
 lib/compressor_libzstd.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/lib/compressor_libzstd.c b/lib/compressor_libzstd.c
index c475077..f47635e 100644
--- a/lib/compressor_libzstd.c
+++ b/lib/compressor_libzstd.c
@@ -123,10 +123,10 @@ 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 = 1 << ilog2(dict_size);
+			dict_size = 1ULL << ilog2(dict_size);
 		}
 	}
-	if (dict_size != 1 << ilog2(dict_size) ||
+	if (dict_size != 1ULL << ilog2(dict_size) ||
 	    dict_size > Z_EROFS_ZSTD_MAX_DICT_SIZE) {
 		erofs_err("invalid dictionary size %u", dict_size);
 		return -EINVAL;
-- 
2.51.0

Re: [PATCH] erofs-utils: lib: fix undefined behavior in zstd dict_size bit shift

[Yifan Zhao]

On 2/22/2026 1:02 PM, Nithurshen wrote:
> cppcheck static analysis flags that shifting the signed 32-bit literal
> `1` by `ilog2(dict_size)` can lead to undefined behavior if the shift
> amount reaches or exceeds 31.
>
> This patch casts the literal to `1ULL` to ensure the shift operates
> safely on an unsigned 64-bit integer, preventing potential overflows
> on different architectures.
>
> Signed-off-by: Nithurshen <nithurshen@gmail.com>
> ---
>   lib/compressor_libzstd.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/lib/compressor_libzstd.c b/lib/compressor_libzstd.c
> index c475077..f47635e 100644
> --- a/lib/compressor_libzstd.c
> +++ b/lib/compressor_libzstd.c
> @@ -123,10 +123,10 @@ 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 = 1 << ilog2(dict_size);
> +			dict_size = 1ULL << ilog2(dict_size);

Hi Nithurshen,


Thank you for catching this. I think using '1U' rather than '1ULL' is 
enough here and below.


Yifan Zhao

>   		}
>   	}
> -	if (dict_size != 1 << ilog2(dict_size) ||
> +	if (dict_size != 1ULL << ilog2(dict_size) ||
>   	    dict_size > Z_EROFS_ZSTD_MAX_DICT_SIZE) {
>   		erofs_err("invalid dictionary size %u", dict_size);
>   		return -EINVAL;

[PATCH v2] erofs-utils: lib: fix undefined behavior in zstd dict_size bit shift

[Nithurshen]

cppcheck static analysis flags that shifting the signed 32-bit literal
`1` by `ilog2(dict_size)` can lead to undefined behavior if the shift
amount reaches or exceeds 31.

This patch casts the literal to `1U` to ensure the shift operates
safely on an unsigned 32-bit integer, preventing potential overflows
on different architectures.

Signed-off-by: Nithurshen <nithurshen@gmail.com>
---
 lib/compressor_libzstd.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/lib/compressor_libzstd.c b/lib/compressor_libzstd.c
index c475077..6330f44 100644
--- a/lib/compressor_libzstd.c
+++ b/lib/compressor_libzstd.c
@@ -123,10 +123,10 @@ 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 = 1 << ilog2(dict_size);
+			dict_size = 1U << ilog2(dict_size);
 		}
 	}
-	if (dict_size != 1 << ilog2(dict_size) ||
+	if (dict_size != 1U << ilog2(dict_size) ||
 	    dict_size > Z_EROFS_ZSTD_MAX_DICT_SIZE) {
 		erofs_err("invalid dictionary size %u", dict_size);
 		return -EINVAL;
-- 
2.51.0