[PATCH v8 03/10] Remove __morecore and __default_morecore

[Adhemerval Zanella]

On 14/07/2021 14:15, Carlos O'Donell via Libc-alpha wrote:
> On 7/14/21 12:42 PM, Guillaume Morin wrote:
>> That said, I was personally aware since it was pointed out on a
>> libhugetlbfs github issue that it was deprecated in 2.32. I think the
>> hope was it would be replaced by something usable, and users would not
>> be left with *no* solution (I was also quietly hoping Eric Munson, the
>> libhugetlbfs maintainer would be reaching out). But if you remove it, I
>> am not quite sure what you expect libhugetlbfs users to do, really.
> 
> Please work with upstream libhugetlbfs to prioritize issue #52.
> 
> Deprecation of morecore in glibc #52 (2020-08-27):
> https://github.com/libhugetlbfs/libhugetlbfs/issues/52
> 

I agree with Carlos and Siddhesh rationale that a dangling function
pointer in an essential code as malloc is a security liability and it 
is about time to get removed (as there is PoC out there that does it to 
exploit glibc malloc). However I think we can work towards a solution to
enable within glibc instead of pushing the support on libhugetlfs, which 
does *not* aim to be a malloc replacement but rather a way to explicit
provide and manage large pages.

There is some discussion last year about providing large pages support
directly on glibc without resorting to THP [1].  The idea is to hook
up the mmap/madvise with the required flags, which seems similar of
what is required by libhugetls morecore() implementation [2]. So I think
it should be feasible to add support for the required bits on glibc
and provide a tunable to actually use it. 

The proposed patchset does require some additional work, such as providing
is through a tunable, allowing different sizes depending of the architecture,
and maybe just dump the sbrk change in favor of just using mmap() for the 
case of large pages.

Guillaume, would this be suffice for your use cases.  The libhugetlsfs
projects does provide more functionality than glibc scope, for instance
a linker script to for bss/data and exe section to large parges, so I
glibc support would be a complement to what libhugetls provides.

[1] https://sourceware.org/pipermail/libc-alpha/2020-May/113539.html
[2] https://github.com/libhugetlbfs/libhugetlbfs/blob/master/morecore.c