netdev - Re: Curious bpf regression in 5.18 already fixed in stable 5.18.3

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Date:   Thu, 16 Jun 2022 09:41:04 -0700
From:   Maciej Żenczykowski <maze@...gle.com>
To:     Stanislav Fomichev <sdf@...gle.com>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Linux NetDev <netdev@...r.kernel.org>,
        BPF Mailing List <bpf@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Sasha Levin <sashal@...nel.org>,
        Carlos Llamas <cmllamas@...gle.com>,
        YiFei Zhu <zhuyifei@...gle.com>
Subject: Re: Curious bpf regression in 5.18 already fixed in stable 5.18.3

On Thu, Jun 16, 2022 at 8:57 AM Stanislav Fomichev <sdf@...gle.com> wrote:
> On Wed, Jun 15, 2022 at 6:36 PM Maciej Żenczykowski <maze@...gle.com> wrote:
> > I'm guessing this means the regression only affects 64-bit archs,
> > where long = void* is 8 bytes > u32 of 4 bytes, but not 32-bit ones,
> > where long = u32 = 4 bytes
> >
> > Unfortunately my dev machine's 32-bit build capability has somehow
> > regressed again and I can't check this.
>
> Seems so, yes. But I'm actually not sure whether we should at all
> treat it as a regression. There is a question of whether that EPERM is
> UAPI or not. That's why we most likely haven't caught it in the
> selftests; most of the time we only check that syscall has returned -1
> and don't pay attention to the particular errno.

EFAULT seems like a terrible error to return no matter what, it has a very clear
'memory read/write access violation' semantic (ie. if you'd done from
userspace you'd get a SIGSEGV)

I'm actually surprised to learn you return EFAULT on positive number...
It should rather be some unique error code or EINVAL or something.

I know someone will argue that (most/all) system calls can return EFAULT...
But that's not actually true.  From a userspace developer the expectation is
they will not return EFAULT if you pass in memory you know is good.

#include <sys/utsname.h>
int main() {
  struct utsname uts;
  uname(&uts);
}

The above cannot EFAULT in spite of it being documented as the only
error uname can report,
because obviously the uts structure on the stack is valid memory.

Maybe ENOSYS would at least make it obvious something is very weird.