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

From: Ingo Molnar
Date: Tue Aug 25 2015 - 05:17:54 EST

* Ingo Molnar <mingo@xxxxxxxxxx> wrote:

> * Johannes Berg <johannes@xxxxxxxxxxxxxxxx> wrote:
> > 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.
> Absolutely, I suggested this as well earlier today, as the scheduler would like
> to make use of it in syscalls with extensible ABIs, such as sched_setattr().
> If people really like this then we could go farther as well and add a standalone
> 'extended errors system call' as well (SyS_errno_extended_get()), which would
> allow the recovery of error strings even for system calls that are not easily
> extensible. We could cache the last error description in the task struct.

If we do that then we don't even have to introduce per system call error code
conversion, but could unconditionally save the last extended error info in the
task struct and continue - this could be done very cheaply with the linker trick
driven integer ID.

I.e. system calls could opt in to do:

return err_str(-EBUSY, "perf/x86: BTS conflicts with active events");

and the overhead of this would be minimal, we'd essentially do something like this
to save the error:

current->err_code = code;

where 'code' is a build time constant in essence.

We could use this even in system calls where the error path is performance
critical, as all the string recovery and copying overhead would be triggered by
applications that opt in via the new system call:

struct err_desc {
const char *message;
const char *owner;
const int code;

SyS_err_get_desc(struct err_desc *err_desc __user);

[ Which could perhaps be a prctl() extension as well (PR_GET_ERR_DESC): finally
some truly matching functionality for prctl(). ]



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