Re: [PATCH 3/3] signalfd: add ability to read siginfo-s without dequeuing signals (v2)

On Thu, Feb 7, 2013 at 10:13 PM, Andrey Wagin [off-list ref] wrote:

2013/2/7 Oleg Nesterov [off-list ref]:

quoted

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:

[...]

quoted

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.

Suppose that the task forks after sys_signalfd(). Now if parent or child
do llseek this affects them both. This is insane because signalfd is
"strange" to say at least, fork/dup/etc inherits signalfd_ctx but not the
"source" of the data.

(Good catch, Oleg.)

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?

From an interface perspective I have no problem with limiting the API

to allow just pread(). If we later decide that there is some way that
the semantics using read() + lseek() could be sensible (which seems
unlikely), we could relax things and allow read().

[...]

Oleg, thank you for the comments. I'm waiting an answer on the
question and after that I'm going to send a final version.

It would be nice if the new patch series has a changelog of the
changes to date, and also includes a fairly detailed description of
the user-space API. Would that be possible?

Thanks,

Michael


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Author of "The Linux Programming Interface"; http://man7.org/tlpi/