Re: [PATCH 0/4] RFC samples converting VLA to alloca

[Tyler Retzlaff <roretzla@linux.microsoft.com>]

On Wed, Apr 10, 2024 at 09:32:10AM +0200, Mattias Rönnblom wrote:
> On 2024-04-08 17:53, Morten Brørup wrote:
> >>From: Tyler Retzlaff [mailto:roretzla@linux.microsoft.com]
> >>Sent: Monday, 8 April 2024 17.27
> >>
> >>For next technboard meeting.
> >>
> >>On Sun, Apr 07, 2024 at 10:03:06AM -0700, Stephen Hemminger wrote:
> >>>On Sun, 7 Apr 2024 13:07:06 +0200
> >>>Morten Brørup <mb@smartsharesystems.com> wrote:
> >>>
> >>>>>From: Mattias Rönnblom [mailto:hofors@lysator.liu.se]
> >>>>>Sent: Sunday, 7 April 2024 11.32
> >>>>>
> >>>>>On 2024-04-04 19:15, Tyler Retzlaff wrote:
> >>>>>>This series is not intended for merge.  It insteat provides examples
> >>>>>of
> >>>>>>converting use of VLAs to alloca() would look like.
> >>>>>>
> >>>>>>what's the advantages of VLA over alloca()?
> >>>>>>
> >>>>>>* sizeof(array) works as expected.
> >>>>>>
> >>>>>>* multi-dimensional arrays are still arrays instead of pointers to
> >>>>>>    dynamically allocated space. this means multiple subscript syntax
> >>>>>>    works (unlike on a pointer) and calculation of addresses into
> >>>>>allocated
> >>>>>>    space in ascending order is performed by the compiler instead of
> >>>>>manually.
> >>>>>>
> >>>>>
> >>>>>alloca() is a pretty obscure mechanism, and also not a part of the C
> >>>>>standard. VLAs are C99, and well-known and understood, and very
> >>>>>efficient.
> >>>>
> >>>>The RFC fails to mention why we need to replace VLAs with something else:
> >>>>
> >>>>VLAs are C99, but not C++; VLAs were made optional in C11.
> >>>>
> >>>>MSVC doesn't support VLAs, and is not going to:
> >>>>https://devblogs.microsoft.com/cppblog/c11-and-c17-standard-support-
> >>arriving-in-msvc/#variable-length-arrays
> >>>>
> >>>>
> >>>>I dislike alloca() too, and the notes section in the alloca(3) man page
> >>even discourages the use of alloca():
> >>>>https://man7.org/linux/man-pages/man3/alloca.3.html
> >>>>
> >>>>But I guess alloca() is the simplest replacement for VLAs.
> >>>>This RFC patch series opens the discussion for alternatives in different
> >>use cases.
> >>>>
> >>>
> >>>The other issue with VLA's is that if the number is something that can be
> >>externally
> >>>input, then it can be a source of stack overflow bugs. That is why the Linux
> >>kernel
> >>>has stopped using them; for security reasons. DPDK has much less of a
> >>security
> >>>trust domain. Mostly need to make sure that no data from network is being
> >>>used to compute VLA size.
> >>>
> >>
> >>Looks like we need to discuss this at the next techboard meeting.
> >>
> >>* MSVC doesn't support C11 optional VLAs (and never will).
> >>* alloca() is an alternative that is available on all platforms/toolchain
> >>   combinations.
> >>* it's reasonable for some VLAs to be turned into regular arrays but it
> >>   would be unsatisfactory to be stuck waiting discussions of defining new
> >>   constant expression macros on a per-use basis.
> >
> >We must generally stop using VLAs, for many reasons.
> 
> What reasons would that be? And which of those reasons are not also
> reasons to stop using alloca().

truncated the sentence, probably should have said where static array is
not practical.