Error injection can only fail a bio today.  This adds a delay_us option so
that a rule can hold a bio back first, to model a slow device.

The delay happens above the driver, so it is invisible to the I/O
statistics and never reaches the blk-mq timeout handler or SCSI error
handling.  What it does exercise is the code waiting above the block
layer: io_uring cancellation, hung task detection, and filesystem or
userspace timeouts.

Two things are worth a look.  A delayed bio is marked with a new
BIO_ERROR_INJECTED bio flag, so the rules are not applied to it again and
it can never pick up a status from another rule.  Resubmitting it below the
injection hook is not enough on its own, because a bio is split below the
hook and the remainder is resubmitted above it.  And holding a bio back
reorders it against bios submitted later, which breaks sequential write
ordering on zoned devices.  Both are documented in patch 3.

Patch 1 is a prep cleanup.  It moves the rejection of an unknown status
tag into the parser, because patch 2 makes a rule without a status valid.

v1: https://lore.kernel.org/linux-block/20260827000115.128093-1-haris.iqbal@linux.dev/

Changes since v1:

 - Add the BIO_ERROR_INJECTED bio flag.  v1 only resubmitted a delayed bio
   below the injection hook, which left the remainder of a split going
   through the hook and matching the same rule again, so a bio was held
   once per split instead of once.

 - Patch 1: tag_to_blk_status() returns an error and passes the status back
   through a pointer, instead of returning BLK_STS_OK for both the "OK" tag
   and an unknown one.  A delay-only rule has no status, so the two have to
   be told apart.

 - Documentation: a delayed bio is held once rather than once per split.

Tested in a VM.

Md Haris Iqbal (3):
  block: reject unknown status tags in error injection rules
  block: allow error injection rules to delay bios
  Documentation: block: document error injection delays

 Documentation/block/error-injection.rst |  58 ++++++++-
 block/blk-core.c                        |  27 ++--
 block/blk.h                             |   3 +-
 block/error-injection.c                 | 165 +++++++++++++++++++++---
 block/error-injection.h                 |   1 +
 include/linux/blk_types.h               |   1 +
 6 files changed, 222 insertions(+), 33 deletions(-)

-- 
2.53.0