linux-kernel - [PATCH 08/10] ioctl_userfaultfd.2: clarify the state of the uffdio

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Date:   Tue, 19 Sep 2023 12:02:04 -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 08/10] ioctl_userfaultfd.2: clarify the state of the
 uffdio_api structure on error

The old FIXME noted that the zeroing was done to differentiate the two
EINVAL cases. It's possible something like this was true historically,
but in current Linux we zero it in *both* EINVAL cases, so this is at
least no longer true.

After reading the code, I can't determine any clear reason why we zero
it in some cases but not in others. So, some simple advice we can give
userspace is: if an error occurs, treat the contents of the structure as
unspecified. Just re-initialize it before retrying UFFDIO_API again.

Signed-off-by: Axel Rasmussen <axelrasmussen@...gle.com>
---
 man2/ioctl_userfaultfd.2 | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/man2/ioctl_userfaultfd.2 b/man2/ioctl_userfaultfd.2
index 1aa9654be..29dca1f6b 100644
--- a/man2/ioctl_userfaultfd.2
+++ b/man2/ioctl_userfaultfd.2
@@ -272,6 +272,14 @@ operation returns 0 on success.
 On error, \-1 is returned and
 .I errno
 is set to indicate the error.
+If an error occurs,
+the kernel may zero the provided
+.I uffdio_api
+structure.
+The caller should treat its contents as unspecified,
+and reinitialize it before re-attempting another
+.B UFFDIO_API
+call.
 Possible errors include:
 .TP
 .B EFAULT
@@ -305,14 +313,6 @@ twice,
 the first time with no features set,
 is explicitly allowed
 as per the two-step feature detection handshake.
-.\" 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.
-.\"
-.\" Mike Rapoport:
-.\"     In my understanding the uffdio_api
-.\"     structure is zeroed to allow the caller
-.\"     to distinguish the reasons for -EINVAL.
-.\"
 .SS UFFDIO_REGISTER
 (Since Linux 4.3.)
 Register a memory address range with the userfaultfd object.
-- 
2.42.0.459.ge4e396fd5e-goog