Re: [PATCH v2 0/4] Fix up bpf_jit_limit some more

On Wed, 13 Oct 2021 at 20:56, Jakub Sitnicki [off-list ref] wrote:

On Tue, Oct 12, 2021 at 03:59 PM CEST, Lorenz Bauer wrote:

quoted

Some more cleanups around bpf_jit_limit to make it readable via sysctl.

Jakub raised the point that a sysctl toggle is UAPI and therefore
can't be easily changed later on. I tried to find another place to stick
the info, but couldn't find a good one. All the current BPF knobs are in
sysctl.

There are examples of read only sysctls:
$ sudo find /proc/sys -perm 0444 | wc -l
90

There are no examples of sysctls with mode 0400 however:
$ sudo find /proc/sys -perm 0400 | wc -l
0

Thoughts?

I threw this idea out there during LPC already, that it would be cool to
use BPF iterators for that. Pinned/preloaded iterators were made for
dumping kernel data on demand after all.

What is missing is a BPF iterator type that would run the program just
once (there is just one thing to print), and a BPF helper to lookup
symbol's address.

I thought this would require a bit of work, but actually getting a PoC
(see below) to work was rather pleasntly straightforward.

Perhaps a bit of a hack but I'd consider it as an alternative.

I spoke to Jakub, I won't have time to work on this myself. So I'll
drop this patch from the series and send a v3 with just the fixes to
bpf_jit_limit.

-- 
Lorenz Bauer  |  Systems Engineer
6th Floor, County Hall/The Riverside Building, SE1 7PB, UK

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