Re: Should we add a compiler warning for large stack frames?

On 2024-04-11 Th 15:01, Andres Freund wrote:
> Hi,
>
> d8f5acbdb9b made me wonder if we should add a compiler option to warn when
> stack frames are large.  gcc compatible compilers have -Wstack-usage=limit, so
> that's not hard.
>
> Huge stack frames are somewhat dangerous, as they can defeat our stack-depth
> checking logic. There are also some cases where large stack frames defeat
> stack-protector logic by compilers/libc/os.
>
> It's not always obvious how large the stack will be. Even if you look at all
> the sizes of the variables defined in a function, inlining can increase that
> substantially.
>
> Here are all the cases a limit of 64k finds.
>
>
> [1345/1940 42  69%] Compiling C object src/bin/pg_verifybackup/pg_verifybackup.p/pg_verifybackup.c.o
> ../../../../../home/andres/src/postgresql/src/bin/pg_verifybackup/pg_verifybackup.c: In function 'verify_file_checksum':
> ../../../../../home/andres/src/postgresql/src/bin/pg_verifybackup/pg_verifybackup.c:842:1: warning: stack usage is 131232 bytes [-Wstack-usage=]
>    842 | verify_file_checksum(verifier_context *context, manifest_file *m,
>        | ^~~~~~~~~~~~~~~~~~~~
>
This one's down to me. I asked Robert some time back why we were using a 
very conservative buffer size, and he agreed we could probably make it 
larger, but the number chosen is mine, not his. It was a completely 
arbitrary choice.

I'm happy to reduce it, but it's not clear to me why we care that much 
for a client binary. There's no stack depth checking going on here.

cheers

andrew

--
Andrew Dunstan
EDB:https://www.enterprisedb.com