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SubjectRe: PIDs limited to 15 significant bits

Just to clarify, the old struct msqid_ds et. al are still used by all
Linux software before glibc 2.2. As of glibc 2.2, there is a new
user-level ABI for the SysV IPC functions {msg,sem,shm}*(), which provides
32-bit pids and 32-bit uids. All software using SysV IPC will need to be
recompiled under glibc 2.2 to take advantage of the new structures.

I don't think that returning incorrect pids to older programs here is
going to be any worse than returning incorrect uids or gids. But, we
should fix the kernel to return (-1) in 16 bits, or something like that,
in case of a pid overflow, to programs using the older structures.

-Chris Wing

> (ii) There is also a rather obscure place in SYSV IPC where a 16-bit pid_t
> is used for the fields msg_lspid and msg_lrpid of the (obsolete)
> struct msqid_ds and the fields shm_cpid and shm_lpid of the (obsolete)
> struct shmid_ds. Nothing compiled today uses this, and almost nothing
> uses it at all, but this nevertheless means that one needs an option
> CONFIG_15BIT_PID (default=N) for backward binary compatibility.
> Andries

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