Re: [PATCH] fs/select: avoid clang stack usage warning

[Andi Kleen]

On 10/6/2022 3:21 PM, Nick Desaulniers wrote:
> On Thu, Mar 07, 2019 at 10:01:36AM +0100, Arnd Bergmann wrote:
> > The select() implementation is carefully tuned to put a sensible amount
> > of data on the stack for holding a copy of the user space fd_set,
> > but not too large to risk overflowing the kernel stack.
> >
> > When building a 32-bit kernel with clang, we need a little more space
> > than with gcc, which often triggers a warning:
> >
> > fs/select.c:619:5: error: stack frame size of 1048 bytes in function 'core_sys_select' [-Werror,-Wframe-larger-than=]
> > int core_sys_select(int n, fd_set __user *inp, fd_set __user *outp,
> >
> > I experimentally found that for 32-bit ARM, reducing the maximum
> > stack usage by 64 bytes keeps us reliably under the warning limit
> > again.
> >
> > Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> > ---
> >   include/linux/poll.h | 4 ++++
> >   1 file changed, 4 insertions(+)
> >
> > diff --git a/include/linux/poll.h b/include/linux/poll.h
> > index 7e0fdcf905d2..1cdc32b1f1b0 100644
> > --- a/include/linux/poll.h
> > +++ b/include/linux/poll.h
> > @@ -16,7 +16,11 @@
> >   extern struct ctl_table epoll_table[]; /* for sysctl */
> >   /* ~832 bytes of stack space used max in sys_select/sys_poll before allocating
> >      additional memory. */
> > +#ifdef __clang__
> > +#define MAX_STACK_ALLOC 768
> Hi Arnd,
> Upon a toolchain upgrade for Android, our 32b x86 image used for
> first-party developer VMs started tripping -Wframe-larger-than= again
> (thanks -Werror) which is blocking our ability to upgrade our toolchain.


I wonder if there is a way to disable the warning or increase the 
threshold just for this function. I don't think attribute optimize would 
work, but perhaps some pragma?


-Andi