linux-kernel - [PATCH v2 2/5] ioctl_userfaultfd.2: correct and update UFFDIO

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Message-ID: <20231003194547.2237424-3-axelrasmussen@google.com>
Date:   Tue,  3 Oct 2023 12:45:44 -0700
From:   Axel Rasmussen <axelrasmussen@...gle.com>
To:     Alejandro Colomar <alx@...nel.org>, Peter Xu <peterx@...hat.com>
Cc:     linux-man@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Axel Rasmussen <axelrasmussen@...gle.com>
Subject: [PATCH v2 2/5] ioctl_userfaultfd.2: correct and update UFFDIO_API
 ioctl error codes

First, it is not correct that repeated UFFDIO_API calls result in
EINVAL. This is true *if both calls enable features*, but in the case
where we're doing a two-step feature detection handshake, the kernel
explicitly expects 2 calls (one with no features set). So, correct this
description.

Then, some new error cases have been added to the kernel recently, and
the man page wasn't updated to note these. So, add in descriptions of
these new error cases.

Signed-off-by: Axel Rasmussen <axelrasmussen@...gle.com>
---
 man2/ioctl_userfaultfd.2 | 24 +++++++++++++++++++-----
 1 file changed, 19 insertions(+), 5 deletions(-)

diff --git a/man2/ioctl_userfaultfd.2 b/man2/ioctl_userfaultfd.2
index ef352a69d..28dd2fcdd 100644
--- a/man2/ioctl_userfaultfd.2
+++ b/man2/ioctl_userfaultfd.2
@@ -256,17 +256,31 @@ refers to an address that is outside the calling process's
 accessible address space.
 .TP
 .B EINVAL
-The userfaultfd has already been enabled by a previous
-.B UFFDIO_API
-operation.
-.TP
-.B EINVAL
 The API version requested in the
 .I api
 field is not supported by this kernel, or the
 .I features
 field passed to the kernel includes feature bits that are not supported
 by the current kernel version.
+.TP
+.B EINVAL
+A previous
+.B UFFDIO_API
+call already enabled one or more features for this userfaultfd.
+Calling
+.B UFFDIO_API
+twice,
+the first time with no features set,
+is explicitly allowed
+as per the two-step feature detection handshake.
+.TP
+.B EPERM
+The
+.B UFFD_FEATURE_EVENT_FORK
+feature was enabled,
+but the calling process doesn't have the
+.B CAP_SYS_PTRACE
+capability.
 .\" FIXME In the above error case, the returned 'uffdio_api' structure is
 .\" zeroed out. Why is this done? This should be explained in the manual page.
 .\"
-- 
2.42.0.609.gbb76f46606-goog