MFC r281436 (by mjg@):

fd: remove filedesc argument from fdclose

Just accept a thread instead. This makes it consistent with fdalloc.

No functional changes.

MFC r269023,r272503,r272505,r272523,r272567,r272569,r272574

Prepare fget_unlocked for reading fd table only once.

Some capsicum functions accept fdp + fd and lookup fde based on that.
Add variants which accept fde.

===============================

Add sequence counters with memory barriers.

Current implementation is somewhat simplistic and hackish,
will be improved later after possible memory barrier overhaul.

===============================

Plug capability races.

fp and appropriate capability lookups were not atomic, which could result in
improper capabilities being checked.

This could result either in protection bypass or in a spurious ENOTCAPABLE.

Make fp + capability check atomic with the help of sequence counters.

===============================

Put and #ifdef _KERNEL around the #include for opt_capsicum.h to
hopefully allow the build to finish after r272505.

===============================

filedesc: fix up breakage introduced in 272505

Include sequence counter supports incoditionally [1]. This fixes reprted build
problems with e.g. nvidia driver due to missing opt_capsicum.h.

Replace fishy looking sizeof with offsetof. Make fde_seq the last member in
order to simplify calculations.

===============================

Keep struct filedescent comments within 80-char limit.

===============================

seq_t needs to be visible to userspace

MFC r268001:
Make fdunshare accept only td parameter.

Proc had to match the thread anyway and 2 parameters were inconsistent
with the rest.

MFC r267710:

fd: replace fd_nfiles with fd_lastfile where appropriate

fd_lastfile is guaranteed to be the biggest open fd, so when the intent
is to iterate over active fds or lookup one, there is no point in looking
beyond that limit.

Few places are left unpatched for now.

MFC r269023,r272503,r272505,r272523,r272567,r272569,r272574

Prepare fget_unlocked for reading fd table only once.

Some capsicum functions accept fdp + fd and lookup fde based on that.
Add variants which accept fde.

===============================

Add sequence counters with memory barriers.

Current implementation is somewhat simplistic and hackish,
will be improved later after possible memory barrier overhaul.

===============================

Plug capability races.

fp and appropriate capability lookups were not atomic, which could result in
improper capabilities being checked.

This could result either in protection bypass or in a spurious ENOTCAPABLE.

Make fp + capability check atomic with the help of sequence counters.

===============================

Put and #ifdef _KERNEL around the #include for opt_capsicum.h to
hopefully allow the build to finish after r272505.

===============================

filedesc: fix up breakage introduced in 272505

Include sequence counter supports incoditionally [1]. This fixes reprted build
problems with e.g. nvidia driver due to missing opt_capsicum.h.

Replace fishy looking sizeof with offsetof. Make fde_seq the last member in
order to simplify calculations.

===============================

Keep struct filedescent comments within 80-char limit.

===============================

seq_t needs to be visible to userspace

MFC r268001:
Make fdunshare accept only td parameter.

Proc had to match the thread anyway and 2 parameters were inconsistent
with the rest.

MFC r267710:

fd: replace fd_nfiles with fd_lastfile where appropriate

fd_lastfile is guaranteed to be the biggest open fd, so when the intent
is to iterate over active fds or lookup one, there is no point in looking
beyond that limit.

Few places are left unpatched for now.