Messages in this thread Patch in this message | | | Date | Thu, 21 Mar 2019 10:47:26 +0100 | From | Alexander Sverdlin <> | Subject | Re: kmsg: lseek errors confuse glibc's dprintf |
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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.
> >>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.) > > and this patch causes a failure that glibc is prepared to accept: > > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c > index 02d6b6d..f6b0c93 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 -ESPIPE; > 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 expecting SEEK_CUR > + * to not yield EINVAL. */ > + ret = -ESPIPE; > + break; > default: > ret = -EINVAL; > } > > Either makes dprintf work, but is either the right solution? > > Thanks. > > Mike.
-- Alexander Sverdlin.
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