[0/2] unrestrict process_madvise() for current process

Message

Lorenzo Stoakes Sept. 23, 2024, 4:03 p.m. UTC

The process_madvise() call was introduced in commit ecb8ac8b1f14
("mm/madvise: introduce process_madvise() syscall: an external memory
hinting API") as a means of performing madvise() operations on another
process.

However, as it provides the means by which to perform multiple madvise()
operations in a batch via an iovec, it is useful to utilise the same
interface for performing operations on the current process rather than a
remote one.

Using this interface targeting the current process is cumbersome - a pidfd
needs to be setup for the current pid, and we are limited to only a subset
of madvise() operations, a limitation sensible for manipulating remote
processes but not meaningful when manipulating the current one.

Commit 22af8caff7d1 ("mm/madvise: process_madvise() drop capability check
if same mm") removed the need for a caller invoking process_madvise() on
its own pidfd to possess the CAP_SYS_NICE capability, however this leaves
the restrictions on operation in place and the cumbersome need for a 'self
pidfd'.

This patch series eliminates both limitations:

1. The restriction on permitted operations is removed when operating
   on the current process.

2. A new flag is introduced - PR_MADV_SELF - which eliminates the need for
   a pidfd - if this flag is set, the pidfd argument is ignored and the
   operation is simply applied to the current process.

Therefore a user can simply invoke:

	process_madvise(0, iovec, n, MADV_..., PR_MADV_SELF);

And perform any madvise() operation they like on the n ranges specified by
the iovec parameter.

This series also introduces a series of self-tests for this feature
asserting that the flag functions as expected.

Lorenzo Stoakes (2):
  mm/madvise: introduce PR_MADV_SELF flag to process_madvise()
  selftests/mm: add test for process_madvise PR_MADV_SELF flag use

 include/uapi/asm-generic/mman-common.h       |   2 +
 mm/madvise.c                                 |  58 +++++++---
 tools/testing/selftests/mm/.gitignore        |   1 +
 tools/testing/selftests/mm/Makefile          |   1 +
 tools/testing/selftests/mm/process_madvise.c | 115 +++++++++++++++++++
 5 files changed, 161 insertions(+), 16 deletions(-)
 create mode 100644 tools/testing/selftests/mm/process_madvise.c

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2.46.0