Messages in this thread | | | Date | Wed, 09 Jan 2013 17:17:41 -0500 | From | Carlos O'Donell <> | Subject | Re: Friendlier EPERM - Request for input |
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On 01/09/2013 04:09 PM, Eric Paris wrote: > On Wed, 2013-01-09 at 21:59 +0100, Jakub Jelinek wrote: >> On Wed, Jan 09, 2013 at 12:53:40PM -0800, Casey Schaufler wrote: >>> I'm suggesting that the string returned by get_extended_error_info() >>> ought to be the audit record the system call would generate, regardless >>> of whether the audit system would emit it or not. >> >> What system call would that info be for and would it be reset on next >> syscall that succeeded, or also failed? >> >> The thing is, various functions e.g. perform some syscall, save errno, do >> some other syscall, and if they decide that the first syscall should be what >> determines the whole function's errno, just restore errno from the saved >> value and return. Similarly, various functions just set errno upon >> detecting some error condition in userspace. >> There is no 1:1 mapping between many libc library calls and syscalls. >> So, when would it be safe to call this new get_extended_error_info function >> and how to determine to which syscall it was relevant?
I asked the same questions as Jakub asked but in a slightly different formulation (http://cygwin.com/ml/libc-alpha/2013-01/msg00267.html).
> I was thinking of it to be the last kernel error. So if the first and > that second operation caused the kernel to want to make available > extended errno information you would end up with the second. I see this > is an informative piece of information, not normative. Not a > replacement for errno. I'm hoping for a best effort way to provide > extended errno information.
IMO Casey's answer is the right solution i.e. whatever the errno behaviour was.
> It would be really neat for libc to have a way to save and restore the > extended errno information, maybe even supply its own if it made the > choice in userspace, but that sounds really hard for the first pass.
Unfortunately without the ability to save/restore the extended information the best you can do is say "You saw an error, here is the last N kernel syscalls you made and their error return codes."
You could take a signal at any time and have interposed syscalls, or you could call a glibc function that makes many syscalls. You need a way to expose the last N syscalls with errors and hope that that's enough information for the user to determine the issue.
> I mean it would be great if we could rewrite every system call with a > cookie so userspace could reliably match things back up, but I just > don't see that as practical. Instead we do the best we can and help > admins and developers most of the time, instead of none of the time.
Agreed.
Cheers, Carlos.
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