On 2023-04-04 01:52, Mingye Wang wrote:
Hi all,
In (somewhat) recent discussions about _FORTIFY_SOURCE level 3, a
common snag to hit seems to be abuse of malloc_usable_size(). The
attached patch is my attempt at making the situation easier to sort
through.
See siddhesh's comment on GitHub.[0] I wonder if the language needs to
be stronger.
[0]: https://github.com/systemd/systemd/issues/22801#issuecomment-1343041481
For more context of my statement, please see this discussion:
https://sourceware.org/pipermail/libc-alpha/2022-November/143599.html
which continued into the next month:
https://sourceware.org/pipermail/libc-alpha/2022-December/143667.html
This amendment that DJ wrote is probably the most precise description of
the current malloc_usage_size situation:
The value returned by malloc_usable_size() may be greater than the
requested size of the allocation because of various internal
implementation details, none of which the programmer should rely on.
This function is intended to only be used for diagnostics and
statistics; writing to the excess memory without first calling
realloc() to resize the allocation is not supported. The returned
value is only valid at the time of the call; any other call to a
malloc family API may invalidate it.
Thanks,
Sid
