Messages in this thread | | | Date | Fri, 8 Feb 2013 20:10:56 +0100 | From | Oleg Nesterov <> | Subject | Re: [PATCH 3/3] signalfd: add ability to read siginfo-s without dequeuing signals (v2) |
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On 02/08, Andrey Wagin wrote: > > 2013/2/7 Oleg Nesterov <oleg@redhat.com>: > > Andrey, sorry for delay. > > > > As for API, I leave this to you and Michael. Not that I like these > > new flags, but I agree that pread() hack was not pretty too. > > > > On 01/29, Andrey Vagin wrote: > >> +static ssize_t signalfd_peek(struct signalfd_ctx *ctx, > >> + siginfo_t *info, loff_t *ppos, int queue_mask) > >> +{ > >> + loff_t seq = *ppos / sizeof(struct signalfd_siginfo); > >> + int signr = 0; > >> + > >> + if (queue_mask & SIGQUEUE_PRIVATE) > >> + signr = peek_signal(¤t->pending, > >> + &ctx->sigmask, info, &seq); > >> + else if (queue_mask & SIGQUEUE_SHARED) > >> + signr = peek_signal(¤t->signal->shared_pending, > >> + &ctx->sigmask, info, &seq); > >> + (*ppos) += sizeof(struct signalfd_siginfo); > > > > Now that this can work even with normal read(), we will actually change > > f_pos. Then perhaps signalfd_fops->llseek() should work too. But this > > is minor... > > lseek works only if FMODE_LSEEK is set. > > You have explained why read&lseek have strange semantics for SIGNALFD_PEEK. > > >Damn. But after I wrote this email I realized that llseek() probably can't > > work. Because peek_offset/f_pos/whatever has to be shared with all processes > > which have this file opened.
Yes. but I thought you decided to ignore this oddity ;)
> So I want to suggest a way how to forbid read() for SIGNALFD_PEEK. > file->f_pos can be initialized to -1. read() returns EINVAL in this > case. In a man page we will write that signals can be dumped only with > help pread(). Is it overload or too ugly?
Well. I do not know. Up to you and Michael.
But honestly, I can't say this all looks really nice. And why do we need SIGNALFD_PEEK then?
Seriously, perhaps we should simply add signalfd_fops->ioctl() for PEEK. Or add PTRACE_{PEEK,POKE}_SIGNAL which looks even logical and useful... And much simpler/straightforward.
But I am not going to argue.
Oleg.
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