[PATCH man-pages v3 3/4] process_madvise.2: fix capability and ptrace requirements

[Zach OKeefe <zokeefe@google.com>]

From: Zach O'Keefe <zokeefe@google.com>

The initial commit of process_madvise(2) to Linux, commit ecb8ac8b1f14
("mm/madvise: introduce process_madvise() syscall: an external memory
hinting API"), relied on PTRACE_MODE_ATTACH_FSCREDS (see ptrace(2)),
but was amended by commit 96cfe2c0fd23 ("mm/madvise: replace ptrace
attach requirement for process_madvise") which replaced this with a
combination of PTRACE_MODE_READ and CAP_SYS_NICE (PTRACE_MODE_READ to
prevent leaking ASLR metadata and CAP_SYS_NICE for influencing process
performance).

The initial commit of process_madvise(2) to man-pages project, made
after the second patch, included two errors:

1) CAP_SYS_ADMIN instead of CAP_SYS_NICE
2) PTRACE_MODE_READ_REALCREDS instead of PTRACE_MODE_READ_FSCREDS

Correct this in the man-page for process_madvise(2).

Fixes: a144f458b ("process_madvise.2: Document process_madvise(2)")
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: Zach O'Keefe <zokeefe@google.com>
---
 man2/process_madvise.2 | 21 +++++++++++++++++----
 1 file changed, 17 insertions(+), 4 deletions(-)

diff --git a/man2/process_madvise.2 b/man2/process_madvise.2
index 6208206e4..44d3b94e8 100644
--- a/man2/process_madvise.2
+++ b/man2/process_madvise.2
@@ -105,16 +105,20 @@ remote process.
 No further elements will be processed beyond that point.
 (See the discussion regarding partial advice in RETURN VALUE.)
 .PP
-Permission to apply advice to another process is governed by a
+.\" commit 96cfe2c0fd23ea7c2368d14f769d287e7ae1082e
+Starting in Linux 5.12,
+permission to apply advice to another process is governed by
 ptrace access mode
-.B PTRACE_MODE_READ_REALCREDS
+.B PTRACE_MODE_READ_FSCREDS
 check (see
 .BR ptrace (2));
 in addition,
 because of the performance implications of applying the advice,
 the caller must have the
-.B CAP_SYS_ADMIN
-capability.
+.B CAP_SYS_NICE
+capability
+(see
+.BR capabilities (7)).
 .SH RETURN VALUE
 On success,
 .BR process_madvise ()
@@ -180,6 +184,15 @@ configuration option.
 The
 .BR process_madvise ()
 system call is Linux-specific.
+.SH NOTES
+When this system call first appeared in Linux 5.10,
+permission to apply advice to another process was entirely governed by
+ptrace access mode
+.B PTRACE_MODE_ATTACH_FSCREDS
+check (see
+.BR ptrace (2)).
+This requirement was relaxed in Linux 5.12 so that the caller didn't require
+full control over the target process.
 .SH SEE ALSO
 .BR madvise (2),
 .BR pidfd_open (2),
-- 
2.38.0.135.g90850a2211-goog