Re: ipv4/tcp.c:4234:1: error: the frame size of 1152 bytes is larger than 1024 bytes [-Werror=frame-larger-than=]

[Shuah Khan <skhan@linuxfoundation.org>]

On 9/7/21 5:14 PM, Linus Torvalds wrote:
> [ Added maintainers for various bits and pieces, since I spent the
> time trying to look at why those bits and pieces wasted stack-space
> and caused problems ]
> 
> On Tue, Sep 7, 2021 at 3:16 PM Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
>>
>> None of these seem to be new.
> 
> That said, all but one of them seem to be (a) very much worth fixing
> and (b) easy to fix.
> 
> The do_tcp_getsockopt() one in tpc.c is a classic case of "lots of
> different case statements, many of them with their own struct
> allocations on stack, and all of them disjoint".
> 
> So the fix for that would be the traditional one of just making helper
> functions for the cases that aren't entirely trivial. We've done that
> before, and it not only fixes stack usage problems, it results in code
> that is easier to read, and generally actually performs better too
> (exactly because it avoids sparse stacks and extra D$ use)
> 
> The pe_test_uints() one is the same horrendous nasty Kunit pattern
> that I fixed in commit 4b93c544e90e ("thunderbolt: test: split up test
> cases in tb_test_credit_alloc_all") that had an even worse case.
> 
> The KUNIT macros create all these individually reasonably small
> initialized structures on stack, and when you have more than a small
> handful of them the KUNIT infrastructure just makes the stack space
> explode. Sometimes the compiler will be able to re-use the stack
> slots, but it seems to be an iffy proposition to depend on it - it
> seems to be a combination of luck and various config options.
> 

I have been concerned about these macros creeping in for a while.
I will take a closer look and work with Brendan to come with a plan
to address it.

> I detest code that exists for debugging or for testing, and that
> violates fundamental rules and causes more problems in the process.
> 

Having recently debugged a problem spinlock hold in an invalid context
related to debug code, I agree with you on this that test and debug code
shouldn't cause problems.

thanks,
-- Shuah