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

[Nick Desaulniers]

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've attached the zstd compressed .config file that reproduces with ToT
LLVM:

$ cd linux
$ zstd -d path/to/config.zst -o .config
$ make ARCH=i386 LLVM=1 -j128 fs/select.o
fs/select.c:625:5: error: stack frame size (1028) exceeds limit (1024)
in 'core_sys_select' [-Werror,-Wframe-larger-than]
int core_sys_select(int n, fd_set __user *inp, fd_set __user *outp,
    ^

As you can see, we're just barely tipping over the limit.  Should I send
a patch to reduce this again? If so, any thoughts by how much?
Decrementing the current value by 4 builds the config in question, but
seems brittle.

Do we need to only do this if !CONFIG_64BIT?
commit ad312f95d41c ("fs/select: avoid clang stack usage warning")
seems to allude to this being more problematic on 32b targets?

> +#else
>  #define MAX_STACK_ALLOC 832
> +#endif
>  #define FRONTEND_STACK_ALLOC	256
>  #define SELECT_STACK_ALLOC	FRONTEND_STACK_ALLOC
>  #define POLL_STACK_ALLOC	FRONTEND_STACK_ALLOC
> -- 
> 2.20.0
> 
>

Attachment: config.zst
Description: application/zstd