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 <nithurs...@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;