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authorLasse Collin <lasse.collin@tukaani.org>2019-05-13 20:05:17 +0300
committerLasse Collin <lasse.collin@tukaani.org>2019-05-13 20:05:17 +0300
commit2a22de439ec63da1927b640eda309296a1e8dce5 (patch)
tree045c0bace711dabf560ceda0223dafe286d7c96d /src/liblzma/common/common.c
parentUpdate THANKS. (diff)
downloadxz-2a22de439ec63da1927b640eda309296a1e8dce5.tar.xz
liblzma: Avoid memcpy(NULL, foo, 0) because it is undefined behavior.
I should have always known this but I didn't. Here is an example as a reminder to myself: int mycopy(void *dest, void *src, size_t n) { memcpy(dest, src, n); return dest == NULL; } In the example, a compiler may assume that dest != NULL because passing NULL to memcpy() would be undefined behavior. Testing with GCC 8.2.1, mycopy(NULL, NULL, 0) returns 1 with -O0 and -O1. With -O2 the return value is 0 because the compiler infers that dest cannot be NULL because it was already used with memcpy() and thus the test for NULL gets optimized out. In liblzma, if a null-pointer was passed to memcpy(), there were no checks for NULL *after* the memcpy() call, so I cautiously suspect that it shouldn't have caused bad behavior in practice, but it's hard to be sure, and the problematic cases had to be fixed anyway. Thanks to Jeffrey Walton.
Diffstat (limited to 'src/liblzma/common/common.c')
-rw-r--r--src/liblzma/common/common.c6
1 files changed, 5 insertions, 1 deletions
diff --git a/src/liblzma/common/common.c b/src/liblzma/common/common.c
index 1399b928..18453ae0 100644
--- a/src/liblzma/common/common.c
+++ b/src/liblzma/common/common.c
@@ -99,7 +99,11 @@ lzma_bufcpy(const uint8_t *restrict in, size_t *restrict in_pos,
const size_t out_avail = out_size - *out_pos;
const size_t copy_size = my_min(in_avail, out_avail);
- memcpy(out + *out_pos, in + *in_pos, copy_size);
+ // Call memcpy() only if there is something to copy. If there is
+ // nothing to copy, in or out might be NULL and then the memcpy()
+ // call would trigger undefined behavior.
+ if (copy_size > 0)
+ memcpy(out + *out_pos, in + *in_pos, copy_size);
*in_pos += copy_size;
*out_pos += copy_size;