Re: [PATCH v2 0/6] perf: Introduce extended syscall error reporting

[Johannes Berg <johannes@sipsolutions.net>]

On Mon, 2015-08-24 at 17:32 +0300, Alexander Shishkin wrote:

> This time around, I employed a linker trick to convert the structures
> containing extended error information into integers, which are then
> made to look just like normal error codes so that IS_ERR_VALUE() and
> friends would still work correctly on them. So no extra pointers in
> the struct perf_event or anywhere else; the extended error codes are
> passed around like normal error codes. They only need to be converted
> in syscalls' topmost return statements. This is done in 1/6.
> 

For the record, as we discussed separately, I'd love to see this move
to more general infrastructure. In wireless (nl80211), for example, we
have a few hundred (!) callsites returning -EINVAL, mostly based on
malformed netlink attributes, and it can be very difficult to figure
out what went wrong; debugging mostly employs a variation of Hugh's
trick.

It would be absolutely necessary to support modules, however that could
get tricky: unless we pass a THIS_MODULE through in some way to
netlink_ack() (in the nl80211 case) we'd have to store the actual index
inside the err_site and assign it on module load somehow. Passing the
module pointer seems better, but has even more wrinkles like what to do
when the error came from a nested call (e.g. nla_parse()) that's in a
different modules ...


Unrelated to all that - maybe perf_errno_to_site() should have some
range checking?

johannes