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 :).