Date: Fri, 17 Apr 2020 15:56:06 +0000 From: Pascal Cuoq <cuoq@...st-in-soft.com> To: "musl@...ts.openwall.com" <musl@...ts.openwall.com> Subject: Invalid pointer subtractions in __shlim and __shgetc ?Hello, both functions `__shlim` and `__shgetc` subtract the members named `buf` and `rpos` of the struct they manipulate. In `__shlim`, this happens in the statement `f->shcnt = f->buf - f->rpos;`. And in `__shgetc`, in happens inside the `shcnt` macro: #define shcnt(f) ((f)->shcnt + ((f)->rpos - (f)->buf)) In our tests, while running `testsuite` in `libc-testsuite`, both the `__shlim` and `__shgetc` functions are reached with `f->buf` non-null and `f->rpos` a null pointer. This can be made visible on execution platforms other than ours by adding a statement at the beginning of the functions: + if (f->buf && !f->rpos) dprintf (2, "XXX Problem in __shlim\n"); + if (f->buf && !f->rpos) dprintf (2, "XXX Problem in __shgetc\n"); Then if, running `libc-testsuite`, you see the following, it means that `f->buf` was non-null and `f->rpos` was null when these points were reached: $ ./testsuite fdopen test passed fcntl test passed fnmatch test passed XXX Problem in __shlim XXX Problem in __shgetc XXX Problem in __shlim XXX Problem in __shgetc XXX Problem in __shlim XXX Problem in __shgetc XXX Problem in __shlim XXX Problem in __shgetc XXX Problem in __shlim XXX Problem in __shgetc XXX Problem in __shlim XXX Problem in __shgetc fscanf test passed (...) This has been tested on the (tag: v1.2.0) branch of git://git.musl-libc.org/musl These pointer subtractions are undefined behavior. This is slightly worse than computing `(char*)0-(char*)0`, which is undefined in C and defined in C++, because compilers for both C and C++ are unlikely to exploit this one for optimization. Subtracting between a non-null pointer and a null pointer on the other hand is undefined behavior in both languages, and it is plausible that doing it may someday have unexpected consequences. I mention this because similar undefined behaviors that were extremely unlikely to cause harm have been fixed in musl in recent months, so that this looks like something you may want to fix too. Pascal Content of type "text/html" skipped
Powered by blists - more mailing lists
Confused about mailing lists and their use? Read about mailing lists on Wikipedia and check out these guidelines on proper formatting of your messages.