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

[Sam James]

Siddhesh Poyarekar <siddhesh@gotplt.org> writes:

> 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.

Honestly, I thought we'd committed that. Oops.

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