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

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Date:   Fri, 7 Oct 2022 18:46:48 -0700
From:   Andi Kleen <ak@...ux.intel.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Arnd Bergmann <arnd@...db.de>
Cc:     linux-fsdevel@...r.kernel.org,
        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, llvm@...ts.linux.dev
Subject: Re: [PATCH] fs/select: avoid clang stack usage warning


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@...db.de>
>> ---
>>   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