linux-kernel - Re: kmsg: lseek errors confuse glibc's dprintf

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Message-ID: <CAK8P3a1gkG6U3zCytY2Ax-DQvExXqG8Z6eGde4W6D7JLa=YwGg@mail.gmail.com>
Date:   Thu, 21 Mar 2019 11:33:33 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Alexander Sverdlin <alexander.sverdlin@...il.com>
Cc:     Mike Crowe <mac@...owe.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kay Sievers <kay@...y.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: kmsg: lseek errors confuse glibc's dprintf

On Thu, Mar 21, 2019 at 10:47 AM Alexander Sverdlin
<alexander.sverdlin@...il.com> wrote:
>
> Hello Mike and all,
>
> On Thu, 15 Jan 2015 17:31:32 +0000
> Mike Crowe <mac@...owe.com> wrote:
>
> > glibc's dprintf implementation does not work correctly with /dev/kmsg file
> > descriptors because glibc treats receiving EBADF and EINVAL from lseek when
> > trying to determine the current file position as errors. See
> > https://sourceware.org/bugzilla/show_bug.cgi?id=17830
>
> we need to conclude on this issue. This is a real bug which is ignored
> for 4 years now. Mike, would you like to re-send a formal patch?
> I can do it as well, preserving a link to your original patch/report.
> In case you'd like to post it yourself, I can be a tester and/or
> provide a reproducer.

The patch needs to be rebased because of the changed file
location. I would also suggest adding a "Cc: stable@...nel.org"
tag so it will get backported into stable kernels.

> > >>From what I can tell prior to Kay Sievers printk record commit
> > e11fea92e13fb91c50bacca799a6131c81929986, calling lseek(fd, 0, SEEK_CUR)
> > with such a file descriptor would not return an error.
> >
> > Prior to Kay's change, Arnd Bergmann's commit
> > 6038f373a3dc1f1c26496e60b6c40b164716f07e seemed to go to some lengths to
> > preserve the successful return code rather than returning (the perhaps more
> > logical) -ESPIPE.
> >
> > glibc is happy with either a successful return or -ESPIPE.
> >
> > For maximum compatibility it seems that success should be returned but
> > given Kay's new seek interface perhaps this isn't helpful.
> >
> > This patch ensures that such a seek succeeds:
> >
> > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> > index 02d6b6d..b3ff6f0 100644
> > --- a/kernel/printk/printk.c
> > +++ b/kernel/printk/printk.c
> > @@ -693,7 +693,7 @@ static loff_t devkmsg_llseek(struct file *file, loff_t offset, int whence)
> >       loff_t ret = 0;
> >
> >       if (!user)
> > -             return -EBADF;
> > +             return (whence == SEEK_CUR) ? 0 : -EBADF;
> >       if (offset)
> >               return -ESPIPE;
> >
> > @@ -718,6 +718,11 @@ static loff_t devkmsg_llseek(struct file *file, loff_t offset, int whence)
> >               user->idx = log_next_idx;
> >               user->seq = log_next_seq;
> >               break;
> > +     case SEEK_CUR:
> > +             /* For compatibility with userspace requesting the
> > +              * current file position. */
> > +             ret = 0;
> > +             break;
> >       default:
> >               ret = -EINVAL;
> >       }
> >
> > (although it could be argued that the !user case should return -ESPIPE
> > rather than EBADF since the file descriptor _is_ valid.)

I don't think the !user case can ever be hit, I would just leave that
to return -BADF and not touch it.

> > @@ -718,6 +718,11 @@ static loff_t devkmsg_llseek(struct file *file, loff_t offset, int whence)
> >               user->idx = log_next_idx;
> >               user->seq = log_next_seq;
> >               break;
> > +     case SEEK_CUR:
> > +             /* For compatibility with userspace expecting SEEK_CUR
> > +              * to not yield EINVAL. */
> > +             ret = -ESPIPE;
> > +             break;
> >       default:
> >               ret = -EINVAL;
> >       }
> >
> > Either makes dprintf work, but is either the right solution?

I'd vote for -ESPIPE, for consistency with the offset!=0 case, but
etiher one is fine with me here.

       Arnd