[RFC PATCH] malloc_usable_size.3: Warn about _FORTIFY_SOURCE interaction

[Siddhesh Poyarekar]

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