lib: fix potential integer underflow on empty render output

The current `uc_render()` implementation uses a `fseek()` call on the
`open_memstream()` provided `FILE *` stream to reserve headroom for the
`uc_string_t` header. The `fseek()` call alone does not guarantee that
the underlying buffer length is updated on all libc implementations though.

This may lead to an integer underflow later on when the `uc_string_t`
header length is substracted from the buffer length after invoking a
template that did not produce any output write operations. In such a
case, a very large value is assigned to `ustr->length` leading to
uninitialized or out-of-bounds memory accesses later on.

Solve this issue by writing the header structure as data using `fwrite()`
which should yield the expected behaviour on all libc environments.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

0 files changed, 0 insertions, 0 deletions