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

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Message-ID: <1440495246.2192.13.camel@sipsolutions.net>
Date:	Tue, 25 Aug 2015 11:34:06 +0200
From:	Johannes Berg <johannes@...solutions.net>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	adrian.hunter@...el.com,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Vince Weaver <vince@...ter.net>,
	Stephane Eranian <eranian@...gle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v2 0/6] perf: Introduce extended syscall error reporting

On Tue, 2015-08-25 at 11:17 +0200, Ingo Molnar wrote:
> 
> 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(). ]
> 
> Hm?

That's neat in a way, but doesn't work in general I think.

Considering the wifi case, or more generally any netlink based
protocol, the syscall (sendmsg) won't return an error, but a subsequent
recvmsg() (which also won't return an error) returns an error message
[in the sense of a protocol message, not a human readable message] to a
buffer provided by the application.
However, this message can be extended relatively easily to include the
string information, but the syscall/prctl wouldn't work since the
syscalls didn't actually fail.

However, it could possibly help with the namespace/module issue if you
also store THIS_MODULE (or perhaps instead a pointer to the module's
error table) in the task. Again not in the netlink case though, I
think, that will always require special handling [although there it
could be stored away in the socket or so, similar to the task]

johannes
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