[LU-13985] seq_file next function must change *pos

• Type: Bug
• Resolution: Fixed
• Priority: Minor
• Fix Version/s: Lustre 2.14.0
• Severity: 3
• Assignee: Neil Brown
• Reporter: Neil Brown

Description

If the .next function used to implement a seq_file ever doesn't change the value of *pos, the result of reading the file can be incorrect.

This specifically includes when the end-of-file is found and NULL is returned.

The error occurs if a 'read()' request is performed while the file pointer is in the middle of the last record, as you could happen if you seek there, or read a block that is not a whole number of records.  In this case the remainder of the last record will be returned followed by a complete copy of that last record.  To avoid the complete copy, *pos must be be changed so that a subsequent call to ->start reports an immediate end of file.
Because this is a common bug, seq_file core code will report a warning if *pos is not changed.

There are several ->next functions in lustre which don't always change *pos.

 

Issue Links

• duplicates: LU-13543 hsm.actions file is broken on RHEL 8.2
• is related to: LU-8066 Move lustre procfs handling to sysfs and debugfs.