https://bugs.llvm.org/show_bug.cgi?id=49923
Bug ID: 49923
Summary: Swapping a closed filebuf runs into undefined behavior
Product: libc++
Version: 11.0
Hardware: All
OS: All
Status: NEW
Severity: normal
Priority: P
Component: All Bugs
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected]
In the following Code UBsan is triggered:
std::ofstream f_old(filename);
assert(f_old << "Hello ");
std::ofstream f_new(filename2);
assert(f_new << "Foo ");
// After swapping both are valid and where they left
f_new.swap(f_old);
assert(f_old << "Bar");
assert(f_new << "World");
f_new.close();
swap(f_new, f_old);
The problem is as follows:
- __extbuf_ is set to some buffer
- during operation of the filebug __extbufnext_ and __extbufend_ are set
relative to that
- filebuf.close calls setbuf(0, 0) which ends up setting __extbuf_ to the
internal min buffer:
https://github.com/llvm/llvm-project/blob/82e537a9d28a2c18bd1637e2eac0e0af658ed829/libcxx/include/fstream#L896
- Note that it does not reset the next/end members
- filebuf.swap then uses the difference between __extbufnext_/__extbufend_ to
__extbuf_ which is invalid as they don't point to the same memory anymore:
https://github.com/llvm/llvm-project/blob/release/12.x/libcxx/include/fstream#L428-L429
- This then triggers UBsan in the following pointer addition with that invalid
offset:
https://github.com/llvm/llvm-project/blob/release/12.x/libcxx/include/fstream#L442-L445
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