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SubjectRe: [PATCH] fs/locks.c fcntl_setlease did not check if a file was opened for writing before granting a read lease
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Jamie Lokier <jamie@shareable.org> writes:

> To detect if anyone has the file open for writing, you'll a new count
> field which keeps track of writer references. Something like this:
>
> if ((arg == F_RDLCK)
> && ((atomic_read(&inode->i_writer_count) != 0)))
>
> You'll also need to modify all the places where that needs to be
> maintained.

Looking at 2.6.0-test11, it seems that its struct inode has such a
field, albeit named i_writecount. Commentary from fs/namei.c:

/*
* get_write_access() gets write permission for a file.
* put_write_access() releases this write permission.
* This is used for regular files.
* We cannot support write (and maybe mmap read-write shared) accesses and
* MAP_DENYWRITE mmappings simultaneously. The i_writecount field of an inode
* can have the following values:
* 0: no writers, no VM_DENYWRITE mappings
* < 0: (-i_writecount) vm_area_structs with VM_DENYWRITE set exist
* > 0: (i_writecount) users are writing to the file.
*
* Normally we operate on that counter with atomic_{inc,dec} and it's safe
* except for the cases where we don't hold i_writecount yet. Then we need to
* use {get,deny}_write_access() - these functions check the sign and refuse
* to do the change if sign is wrong. Exclusion between them is provided by
* spinlock (arbitration_lock) and I'll rip the second arsehole to the first
* who will try to move it in struct inode - just leave it here.
*/
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