Userspace libraries, e.g. glibc's dprintf(), expect the default return value for invalid seek situations: -ESPIPE, but when the IO was over /dev/kmsg the current state of kernel code was returning the generic case of an -EINVAL. Hence, userspace programs were not behaving as expected or documented.
With this patch we add SEEK_CUR case returning the expected value and also a simple mention of it in kernel's documentation for those relying on that for guidance.
Signed-off-by: Bruno Meneguele bmeneg@redhat.com --- Documentation/ABI/testing/dev-kmsg | 2 ++ kernel/printk/printk.c | 4 ++++ 2 files changed, 6 insertions(+)
diff --git a/Documentation/ABI/testing/dev-kmsg b/Documentation/ABI/testing/dev-kmsg index f307506eb54c..8533d28e6fda 100644 --- a/Documentation/ABI/testing/dev-kmsg +++ b/Documentation/ABI/testing/dev-kmsg @@ -56,6 +56,8 @@ Description: The /dev/kmsg character device node provides userspace access seek after the last record available at the time the last SYSLOG_ACTION_CLEAR was issued.
+ While SEEK_CUR sets -ESPIPE (invalid seek) to errno. + The output format consists of a prefix carrying the syslog prefix including priority and facility, the 64 bit message sequence number and the monotonic timestamp in microseconds, diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index ad4606234545..d02606723d2d 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -963,6 +963,10 @@ 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: + /* return the default errno for invalid seek */ + ret = -ESPIPE; + break; default: ret = -EINVAL; }
On Thu, Mar 12, 2020 at 09:35:33PM -0300, Bruno Meneguele wrote:
Userspace libraries, e.g. glibc's dprintf(), expect the default return value for invalid seek situations: -ESPIPE, but when the IO was over /dev/kmsg the current state of kernel code was returning the generic case of an -EINVAL. Hence, userspace programs were not behaving as expected or documented.
With this patch we add SEEK_CUR case returning the expected value and also a simple mention of it in kernel's documentation for those relying on that for guidance.
Signed-off-by: Bruno Meneguele bmeneg@redhat.com
Documentation/ABI/testing/dev-kmsg | 2 ++ kernel/printk/printk.c | 4 ++++ 2 files changed, 6 insertions(+)
<formletter>
This is not the correct way to submit patches for inclusion in the stable kernel tree. Please read: https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html for how to do this properly.
</formletter>
On Fri, Mar 13, 2020 at 08:22:54AM +0100, Greg KH wrote:
On Thu, Mar 12, 2020 at 09:35:33PM -0300, Bruno Meneguele wrote:
Userspace libraries, e.g. glibc's dprintf(), expect the default return value for invalid seek situations: -ESPIPE, but when the IO was over /dev/kmsg the current state of kernel code was returning the generic case of an -EINVAL. Hence, userspace programs were not behaving as expected or documented.
With this patch we add SEEK_CUR case returning the expected value and also a simple mention of it in kernel's documentation for those relying on that for guidance.
Signed-off-by: Bruno Meneguele bmeneg@redhat.com
Documentation/ABI/testing/dev-kmsg | 2 ++ kernel/printk/printk.c | 4 ++++ 2 files changed, 6 insertions(+)
<formletter>
This is not the correct way to submit patches for inclusion in the stable kernel tree. Please read: https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html for how to do this properly.
</formletter>
ouch, yes of course. Sorry for the noise. Will repost it once the concerns with the patch are solved.
Thanks Greg.
On (20/03/12 21:35), Bruno Meneguele wrote:
Userspace libraries, e.g. glibc's dprintf(), expect the default return value for invalid seek situations: -ESPIPE, but when the IO was over /dev/kmsg the current state of kernel code was returning the generic case of an -EINVAL. Hence, userspace programs were not behaving as expected or documented.
Hmm. I don't think I see ESPIPE in documentation [0], [1], [2]
[0] https://pubs.opengroup.org/onlinepubs/9699919799/functions/fprintf.html [1] http://man7.org/linux/man-pages/man3/dprintf.3p.html [2] http://man7.org/linux/man-pages/man3/fprintf.3p.html
-ss
On Fri, Mar 13, 2020 at 04:34:25PM +0900, Sergey Senozhatsky wrote:
On (20/03/12 21:35), Bruno Meneguele wrote:
Userspace libraries, e.g. glibc's dprintf(), expect the default return value for invalid seek situations: -ESPIPE, but when the IO was over /dev/kmsg the current state of kernel code was returning the generic case of an -EINVAL. Hence, userspace programs were not behaving as expected or documented.
Hmm. I don't think I see ESPIPE in documentation [0], [1], [2]
[0] https://pubs.opengroup.org/onlinepubs/9699919799/functions/fprintf.html [1] http://man7.org/linux/man-pages/man3/dprintf.3p.html [2] http://man7.org/linux/man-pages/man3/fprintf.3p.html
-ss
Ok, I poorly expressed the notion of "documentantion". The userspace doesn't tell about returning -ESPIPE, but to the functions work properly they watch for -ESPIPE returning from the syscall. For instance, gblic dprintf() implementation:
dprintf: __vdprintf_internal: _IO_new_file_attach:
if (_IO_SEEKOFF (fp, (off64_t)0, _IO_seek_cur, _IOS_INPUT|_IOS_OUTPUT) == _IO_pos_BAD && errno != ESPIPE) return NULL;
With that, if the seek fails, but return anything other than ESPIPE the dprintf() will also fail returning -EINVAL to dprintf() caller. While if ESPIPE is returned, it's "ignored" and the call still works. The way we have today make kmsg an exception case among the rest of the system files where you can open with dprintf.
One of the things I could agree with is removing the SEEK call from dprintf, since fprintf basically follows the same steps, but doesn't seek anything. But at the same time, IMO it makes sense to make kmsg interface complaint with the errno return values.
From: Bruno Meneguele
Sent: 13 March 2020 11:02 On Fri, Mar 13, 2020 at 04:34:25PM +0900, Sergey Senozhatsky wrote:
On (20/03/12 21:35), Bruno Meneguele wrote:
Userspace libraries, e.g. glibc's dprintf(), expect the default return value for invalid seek situations: -ESPIPE, but when the IO was over /dev/kmsg the current state of kernel code was returning the generic case of an -EINVAL. Hence, userspace programs were not behaving as expected or documented.
Hmm. I don't think I see ESPIPE in documentation [0], [1], [2]
[0] https://pubs.opengroup.org/onlinepubs/9699919799/functions/fprintf.html [1] http://man7.org/linux/man-pages/man3/dprintf.3p.html [2] http://man7.org/linux/man-pages/man3/fprintf.3p.html
-ss
Ok, I poorly expressed the notion of "documentantion". The userspace doesn't tell about returning -ESPIPE, but to the functions work properly they watch for -ESPIPE returning from the syscall. For instance, gblic dprintf() implementation:
dprintf: __vdprintf_internal: _IO_new_file_attach:
if (_IO_SEEKOFF (fp, (off64_t)0, _IO_seek_cur, _IOS_INPUT|_IOS_OUTPUT) == _IO_pos_BAD && errno != ESPIPE) return NULL;
Someone explain why it is doing an explicit seek to the current position? The only reason to do that is to get the current offset.
David
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On Fri, Mar 13, 2020 at 11:06:42AM +0000, David Laight wrote:
From: Bruno Meneguele
Sent: 13 March 2020 11:02 On Fri, Mar 13, 2020 at 04:34:25PM +0900, Sergey Senozhatsky wrote:
On (20/03/12 21:35), Bruno Meneguele wrote:
Userspace libraries, e.g. glibc's dprintf(), expect the default return value for invalid seek situations: -ESPIPE, but when the IO was over /dev/kmsg the current state of kernel code was returning the generic case of an -EINVAL. Hence, userspace programs were not behaving as expected or documented.
Hmm. I don't think I see ESPIPE in documentation [0], [1], [2]
[0] https://pubs.opengroup.org/onlinepubs/9699919799/functions/fprintf.html [1] http://man7.org/linux/man-pages/man3/dprintf.3p.html [2] http://man7.org/linux/man-pages/man3/fprintf.3p.html
-ss
Ok, I poorly expressed the notion of "documentantion". The userspace doesn't tell about returning -ESPIPE, but to the functions work properly they watch for -ESPIPE returning from the syscall. For instance, gblic dprintf() implementation:
dprintf: __vdprintf_internal: _IO_new_file_attach:
if (_IO_SEEKOFF (fp, (off64_t)0, _IO_seek_cur, _IOS_INPUT|_IOS_OUTPUT) == _IO_pos_BAD && errno != ESPIPE) return NULL;
Someone explain why it is doing an explicit seek to the current position? The only reason to do that is to get the current offset.
David
dprintf gets a fd as input and convert it to a FILE structure, with that it can't garantuee the previous state of that fd: was it already manipulated? Thus they check the current position to make sure it's not junk.
But that's me guessing things about a code from 1996 :).
On (20/03/13 08:02), Bruno Meneguele wrote:
Ok, I poorly expressed the notion of "documentantion". The userspace doesn't tell about returning -ESPIPE, but to the functions work properly they watch for -ESPIPE returning from the syscall. For instance, gblic dprintf() implementation:
dprintf: __vdprintf_internal: _IO_new_file_attach:
if (_IO_SEEKOFF (fp, (off64_t)0, _IO_seek_cur, _IOS_INPUT|_IOS_OUTPUT) == _IO_pos_BAD && errno != ESPIPE) return NULL;
With that, if the seek fails, but return anything other than ESPIPE the dprintf() will also fail returning -EINVAL to dprintf() caller. While if ESPIPE is returned, it's "ignored" and the call still works. The way we have today make kmsg an exception case among the rest of the system files where you can open with dprintf.
One of the things I could agree with is removing the SEEK call from dprintf, since fprintf basically follows the same steps, but doesn't seek anything. But at the same time, IMO it makes sense to make kmsg interface complaint with the errno return values.
The code in questions is very old. So let's add the missing bit to the kernel. At the same time, we probably can have a slightly more detailed documentation / code comment.
-ss
On Tue, Mar 17, 2020 at 11:03:26AM +0900, Sergey Senozhatsky wrote:
On (20/03/13 08:02), Bruno Meneguele wrote:
Ok, I poorly expressed the notion of "documentantion". The userspace doesn't tell about returning -ESPIPE, but to the functions work properly they watch for -ESPIPE returning from the syscall. For instance, gblic dprintf() implementation:
dprintf: __vdprintf_internal: _IO_new_file_attach:
if (_IO_SEEKOFF (fp, (off64_t)0, _IO_seek_cur, _IOS_INPUT|_IOS_OUTPUT) == _IO_pos_BAD && errno != ESPIPE) return NULL;
With that, if the seek fails, but return anything other than ESPIPE the dprintf() will also fail returning -EINVAL to dprintf() caller. While if ESPIPE is returned, it's "ignored" and the call still works. The way we have today make kmsg an exception case among the rest of the system files where you can open with dprintf.
One of the things I could agree with is removing the SEEK call from dprintf, since fprintf basically follows the same steps, but doesn't seek anything. But at the same time, IMO it makes sense to make kmsg interface complaint with the errno return values.
The code in questions is very old. So let's add the missing bit to the kernel. At the same time, we probably can have a slightly more detailed documentation / code comment.
-ss
Sure thing.
I'm going to send a v2 of this patch with more details still today or tomorrow.
Thanks!
On Tue, Mar 17, 2020 at 05:54:00AM -0300, Bruno Meneguele wrote:
On Tue, Mar 17, 2020 at 11:03:26AM +0900, Sergey Senozhatsky wrote:
On (20/03/13 08:02), Bruno Meneguele wrote:
Ok, I poorly expressed the notion of "documentantion". The userspace doesn't tell about returning -ESPIPE, but to the functions work properly they watch for -ESPIPE returning from the syscall. For instance, gblic dprintf() implementation:
dprintf: __vdprintf_internal: _IO_new_file_attach:
if (_IO_SEEKOFF (fp, (off64_t)0, _IO_seek_cur, _IOS_INPUT|_IOS_OUTPUT) == _IO_pos_BAD && errno != ESPIPE) return NULL;
With that, if the seek fails, but return anything other than ESPIPE the dprintf() will also fail returning -EINVAL to dprintf() caller. While if ESPIPE is returned, it's "ignored" and the call still works. The way we have today make kmsg an exception case among the rest of the system files where you can open with dprintf.
One of the things I could agree with is removing the SEEK call from dprintf, since fprintf basically follows the same steps, but doesn't seek anything. But at the same time, IMO it makes sense to make kmsg interface complaint with the errno return values.
The code in questions is very old. So let's add the missing bit to the kernel. At the same time, we probably can have a slightly more detailed documentation / code comment.
-ss
Sure thing.
I'm going to send a v2 of this patch with more details still today or tomorrow.
Thanks!
Just fyi, v2 was posted: https://lkml.org/lkml/2020/3/17/321
linux-stable-mirror@lists.linaro.org
-
Bruno Meneguele -
David Laight -
Greg KH -
Sergey Senozhatsky