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

[Stephen Hemminger <stephen@networkplumber.org>]

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.