procfs: fix pthread cross-thread naming if !PR_DUMPABLE

procfs: fix pthread cross-thread naming if !PR_DUMPABLE

The PR_DUMPABLE flag causes the pid related paths of the proc file
system to be owned by ROOT.

The implementation of pthread_set/getname_np however needs access to
/proc/<pid>/task/<tid>/comm.  If PR_DUMPABLE is false this
implementation is locked out.

This patch installs a special permission function for the file "comm"
that grants read and write access to all threads of the same group
regardless of the ownership of the inode.  For all other threads the
function falls back to the generic inode permission check.

[[email protected]: fix spello in comment]
Signed-off-by: Janis Danisevskis <[email protected]>
Acked-by: Kees Cook <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Cyrill Gorcunov <[email protected]>
Cc: Alexey Dobriyan <[email protected]>
Cc: Colin Ian King <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Minfei Huang <[email protected]>
Cc: John Stultz <[email protected]>
Cc: Calvin Owens <[email protected]>
Cc: Jann Horn <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>