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

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Date:   Thu, 7 Mar 2019 08:19:55 -0800
From:   Andi Kleen <ak@...ux.intel.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     linux-fsdevel@...r.kernel.org,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Hellwig <hch@....de>,
        Eric Dumazet <edumazet@...gle.com>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs/select: avoid clang stack usage warning

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.

Could just use 768 bytes unconditionally. I doubt a few bytes more or less
will make too much difference.

Other than that

Reviewed-by: Andi Kleen <ak@...ux.intel.com>

-Andi