Re: [PATCH] kasan: Remove use after scope bugs detection.

On 1/14/19 4:24 PM, Dmitry Vyukov wrote:

On Fri, Jan 11, 2019 at 7:58 PM Andrey Ryabinin [off-list ref] wrote:

quoted

Use after scope bugs detector seems to be almost entirely useless
for the linux kernel. It exists over two years, but I've seen only
one valid bug so far [1]. And the bug was fixed before it has been
reported. There were some other use-after-scope reports, but they
were false-positives due to different reasons like incompatibility
with structleak plugin.

This feature significantly increases stack usage, especially with
GCC < 9 version, and causes a 32K stack overflow. It probably
adds performance penalty too.

Given all that, let's remove use-after-scope detector entirely.

While preparing this patch I've noticed that we mistakenly enable
use-after-scope detection for clang compiler regardless of
CONFIG_KASAN_EXTRA setting. This is also fixed now.

Hi Andrey,

I am on a fence. On one hand removing bug detection sucks and each
case of a missed memory corruption leads to a splash of assorted bug
reports by syzbot. On the other hand everything you said is true.
Maybe support for CONFIG_VMAP_STACK will enable stacks larger then
PAGE_ALLOC_COSTLY_ORDER?

Yes, with vmap stacks higher order won't be a problem, since vmalloc() does only 0-order 
allocations. But even with vmap stacks use-after-scope won't become useful,
thus I don't see the point of re-enabling it with vmap stacks.
If feature doesn't detect bugs, but waste resources, than it's bad for detecting bugs.
We wasting our limited resources for useless checks, instead of using these resources
for doing more useful checks, running tests faster hence detecting more bugs per-time. 





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