Re: [PATCH] Enable '-Werror' by default for all kernel builds

On Tue, Sep 7, 2021 at 2:11 AM Arnd Bergmann [off-list ref] wrote:

quoted

x86_64-alpine.log:drivers/gpu/drm/amd/amdgpu/../display/dc/calcs/dcn_calcs.c:452:13: error: stack frame size (1800) exceeds limit (1280) in function 'dcn_bw_calc_rq_dlg_ttu' [-Werror,-Wframe-larger-than]
x86_64-alpine.log:drivers/gpu/drm/amd/amdgpu/../display/dc/dml/dcn21/display_rq_dlg_calc_21.c:1657:6: error: stack frame size (1336) exceeds limit (1280) in function 'dml21_rq_dlg_get_dlg_reg' [-Werror,-Wframe-larger-than]
x86_64-alpine.log:drivers/gpu/drm/amd/amdgpu/../display/dc/dml/dcn30/display_rq_dlg_calc_30.c:1831:6: error: stack frame size (1352) exceeds limit (1280) in function 'dml30_rq_dlg_get_dlg_reg' [-Werror,-Wframe-larger-than]
x86_64-alpine.log:drivers/gpu/drm/amd/amdgpu/../display/dc/dml/dcn31/display_rq_dlg_calc_31.c:1676:6: error: stack frame size (1336) exceeds limit (1280) in function 'dml31_rq_dlg_get_dlg_reg' [-Werror,-Wframe-larger-than]
x86_64-alpine.log:drivers/vhost/scsi.c:1831:12: error: stack frame size (1320) exceeds limit (1280) in function 'vhost_scsi_release' [-Werror,-Wframe-larger-than]

Another instance where distros lower CONFIG_FRAME_WARN below the 2048
default. Again, none look particularly scary but should still probably
be dealt with.

I would argue that they are still scary and should be addressed in the
code, it's just that we don't see them on build bots that use the 2048 byte default.

No, they are scary for another reason entirely: clang is clearly doing
a *HORRIBLE* job with stack usage.

To take that dml30_rq_dlg_get_dlg_reg() function as an example: yes,
it has a few structures on the stack, but gcc allocates 512-720 bytes
of stack space depending on my config.

Not 1280 bytes.

So it's not even *close* to the 1024 byte limit with gcc, much less the 2kB one.

I don't know why clang basically decides to use almost double the
stack space. Maybe it's some other config option that does it, I tried
a fairly normal one and a "almost everythign enabled" one, and
couldn't get close to the reported stack frame size with gcc.

Just to try to make things as close as possible, I tried with the
exact same normal non-debug config (apart from obvious
compiler-dependent things), and picked that dml30_rq_dlg_get_dlg_reg()
function to look at (for no real reason other than that the stack
frame was biggest above.

Gcc did a 720-byte stack frame for that case. Not great, but whatever.

clang did a 1136-byte stack frame for the same thing.

Do I know why? No. I do note that that code is disgusting.

It's passing one of those structs around by value, for example. That's
a 72-byte structure that is copied on the stack due to stupid calling
conventions. Maybe clang generates a few extra temporaries for it as
part of the function call stack setup? Who knows..

                 Linus