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

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

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.
* there is resistance to using alloca() vs VLA so my proposal is to
  change only the code that is built to target windows.