Re: Segments

David Brown <david.brown@hesbynett.no> wrote:

On 16/01/2025 13:35, Michael S wrote:

On Thu, 16 Jan 2025 12:36:45 +0100
David Brown <david.brown@hesbynett.no> wrote:
 

On 15/01/2025 21:59, Thomas Koenig wrote:

Michael S <already5chosen@yahoo.com> schrieb:

On Wed, 15 Jan 2025 18:00:34 -0000 (UTC)
Thomas Koenig <tkoenig@netcologne.de> wrote:
 

As you can guess, in kernel drivers VLA are unwelcome.

 
I can imagine that they are - but I really don't understand why.  I've
never understood why people think there is something "dangerous" about
VLAs, or why they think using heap allocations is somehow "safer".

VLA normally allocate on the stack.  Which at first glance look
great.  But once one realize how small are stacks in modern
systems (compared to whole memory), this no longer looks good.
Basically, to use VLA one needs rather small bound on maximal
size of array.  Given such bound always allocating maximal
size is simpler.  Without _small_ bound on size heap is
safer, as it is desined to handle also big allocations.

In the past I was a fan of VLA and stack allocation in general.
But I saw enough bug reports due to programs exceeding their
stack limits that I changed my view.

I do not know about Windows, but IIUC in some period Linux limit
for kernel stack was something like 2 kB (single page shared
with some other per-process data structures).  I think it
was increased later, but even moderate size arrays are
unwelcame on kernel stack due to size limits.

VMTs are, may
be, tolerable (I wonder what is current policy of Linux and BSD
kernels), but hardly desirable.

IMO VMT-s are vastly superior to raw pointers, but to fully
get their advantages one would need better tools.  Also,
kernel needs to deal with variable size arrays embedded in
various data structures.  This is possible using pointers,
but current VMT-s are too weak for many such uses.

--
                              Waldek Hebisch