Messages in this thread | | | Date | Thu, 21 Mar 2019 21:14:50 +0000 | From | Mike Crowe <> | Subject | Re: kmsg: lseek errors confuse glibc's dprintf |
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On Thursday 21 March 2019 at 11:33:33 +0100, Arnd Bergmann wrote: > On Thu, Mar 21, 2019 at 10:47 AM Alexander Sverdlin > <alexander.sverdlin@gmail.com> wrote: > > > > Hello Mike and all, > > > > On Thu, 15 Jan 2015 17:31:32 +0000 > > Mike Crowe <mac@mcrowe.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@kernel.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.
I originally wrote the patch for linux-3.8(!) and back then a write-only kmsg instance had no private data. It looks like this changed with 750afe7babd117daabebf4855da18e4418ea845e in v4.8, so user should now always be valid.
> > > @@ -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.
It seems that this is the version we've successfully been running in our kernel since then, so that's good.
Thanks.
Mike.
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