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Re: [PATCH] xenbus: avoid stack overflow warning

On 05.05.20 17:01, Arnd Bergmann wrote:
On Tue, May 5, 2020 at 4:34 PM Jürgen Groß <jgross@xxxxxxxx> wrote:
On 05.05.20 16:15, Arnd Bergmann wrote:
The __xenbus_map_ring() function has two large arrays, 'map' and
'unmap' on its stack. When clang decides to inline it into its caller,
xenbus_map_ring_valloc_hvm(), the total stack usage exceeds the warning
limit for stack size on 32-bit architectures.

drivers/xen/xenbus/xenbus_client.c:592:12: error: stack frame size of 1104 
bytes in function 'xenbus_map_ring_valloc_hvm' [-Werror,-Wframe-larger-than=]

As far as I can tell, other compilers don't inline it here, so we get
no warning, but the stack usage is actually the same. It is possible
for both arrays to use the same location on the stack, but the compiler
cannot prove that this is safe because they get passed to external
functions that may end up using them until they go out of scope.

Move the two arrays into separate basic blocks to limit the scope
and force them to occupy less stack in total, regardless of the
inlining decision.

Why don't you put both arrays into a union?

I considered that as well, and don't really mind either way. I think it does
get a bit ugly whatever we do. If you prefer the union, I can respin the
patch that way.

Hmm, thinking more about it I think the real clean solution would be to
extend struct map_ring_valloc_hvm to cover the pv case, too, to add the
map and unmap arrays (possibly as a union) to it and to allocate it
dynamically instead of having it on the stack.

Would you be fine doing this?




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