[libc++] Fix undefined behavior in `std::filebuf`
Fixes https://github.com/llvm/llvm-project/issues/49267.
Fixes https://github.com/llvm/llvm-project/issues/49282.
Fixes https://github.com/llvm/llvm-project/issues/49789.
Reviewed By: ldionne
Differential Revision: https://reviews.llvm.org/D122257
NOKEYCHECK=True
GitOrigin-RevId: 4ff70dba3839ce9510499a79f93522b67cab504c
diff --git a/include/fstream b/include/fstream
index 907c59e..7608daa 100644
--- a/include/fstream
+++ b/include/fstream
@@ -419,25 +419,38 @@
basic_streambuf<char_type, traits_type>::swap(__rhs);
if (__extbuf_ != __extbuf_min_ && __rhs.__extbuf_ != __rhs.__extbuf_min_)
{
- _VSTD::swap(__extbuf_, __rhs.__extbuf_);
- _VSTD::swap(__extbufnext_, __rhs.__extbufnext_);
- _VSTD::swap(__extbufend_, __rhs.__extbufend_);
+ // Neither *this nor __rhs uses the small buffer, so we can simply swap the pointers.
+ std::swap(__extbuf_, __rhs.__extbuf_);
+ std::swap(__extbufnext_, __rhs.__extbufnext_);
+ std::swap(__extbufend_, __rhs.__extbufend_);
}
else
{
- ptrdiff_t __ln = __extbufnext_ - __extbuf_;
- ptrdiff_t __le = __extbufend_ - __extbuf_;
- ptrdiff_t __rn = __rhs.__extbufnext_ - __rhs.__extbuf_;
- ptrdiff_t __re = __rhs.__extbufend_ - __rhs.__extbuf_;
+ ptrdiff_t __ln = __extbufnext_ ? __extbufnext_ - __extbuf_ : 0;
+ ptrdiff_t __le = __extbufend_ ? __extbufend_ - __extbuf_ : 0;
+ ptrdiff_t __rn = __rhs.__extbufnext_ ? __rhs.__extbufnext_ - __rhs.__extbuf_ : 0;
+ ptrdiff_t __re = __rhs.__extbufend_ ? __rhs.__extbufend_ - __rhs.__extbuf_ : 0;
if (__extbuf_ == __extbuf_min_ && __rhs.__extbuf_ != __rhs.__extbuf_min_)
{
+ // *this uses the small buffer, but __rhs doesn't.
__extbuf_ = __rhs.__extbuf_;
__rhs.__extbuf_ = __rhs.__extbuf_min_;
+ std::memmove(__rhs.__extbuf_min_, __extbuf_min_, sizeof(__extbuf_min_));
}
else if (__extbuf_ != __extbuf_min_ && __rhs.__extbuf_ == __rhs.__extbuf_min_)
{
+ // *this doesn't use the small buffer, but __rhs does.
__rhs.__extbuf_ = __extbuf_;
__extbuf_ = __extbuf_min_;
+ std::memmove(__extbuf_min_, __rhs.__extbuf_min_, sizeof(__extbuf_min_));
+ }
+ else
+ {
+ // Both *this and __rhs use the small buffer.
+ char __tmp[sizeof(__extbuf_min_)];
+ std::memmove(__tmp, __extbuf_min_, sizeof(__extbuf_min_));
+ std::memmove(__extbuf_min_, __rhs.__extbuf_min_, sizeof(__extbuf_min_));
+ std::memmove(__rhs.__extbuf_min_, __tmp, sizeof(__extbuf_min_));
}
__extbufnext_ = __extbuf_ + __rn;
__extbufend_ = __extbuf_ + __re;