This documentation is for F_SEAL_FUTURE_WRITE patches that are in linux-next.
Joel Fernandes (Google) (2):
fcntl.2: Update manpage with new memfd F_SEAL_FUTURE_WRITE seal
memfd_create.2: Update manpage with new memfd F_SEAL_FUTURE_WRITE seal
man2/fcntl.2 | 15 +++++++++++++++
man2/memfd_create.2 | 15 ++++++++++++++-
2 files changed, 29 insertions(+), 1 deletion(-)
--
2.21.0.360.g471c308f928-goog
More details of the seal can be found in the LKML patch:
https://lore.kernel.org/lkml/[email protected]/T/#t
Signed-off-by: Joel Fernandes (Google) <[email protected]>
---
man2/fcntl.2 | 15 +++++++++++++++
1 file changed, 15 insertions(+)
diff --git a/man2/fcntl.2 b/man2/fcntl.2
index fce4f4c2b3bd..e01e2c075b5b 100644
--- a/man2/fcntl.2
+++ b/man2/fcntl.2
@@ -1525,6 +1525,21 @@ Furthermore, if there are any asynchronous I/O operations
.RB ( io_submit (2))
pending on the file,
all outstanding writes will be discarded.
+.TP
+.BR F_SEAL_FUTURE_WRITE
+If this seal is set, the contents of the file can be modified only from
+existing writeable mappings that were created prior to the seal being set.
+Any attempt to create a new writeable mapping on the memfd via
+.BR mmap (2)
+will fail with
+.BR EPERM.
+Also any attempts to write to the memfd via
+.BR write (2)
+will fail with
+.BR EPERM.
+This is useful in situations where existing writable mapped regions need to be
+kept intact while preventing any future writes. For example, to share a
+read-only memory buffer to other processes that only the sender can write to.
.\"
.SS File read/write hints
Write lifetime hints can be used to inform the kernel about the relative
--
2.21.0.360.g471c308f928-goog
More details of the seal can be found in the LKML patch:
https://lore.kernel.org/lkml/[email protected]/T/#t
Signed-off-by: Joel Fernandes (Google) <[email protected]>
---
man2/memfd_create.2 | 15 ++++++++++++++-
1 file changed, 14 insertions(+), 1 deletion(-)
diff --git a/man2/memfd_create.2 b/man2/memfd_create.2
index 15b1362f5525..3b7f032407ed 100644
--- a/man2/memfd_create.2
+++ b/man2/memfd_create.2
@@ -280,7 +280,15 @@ in order to restrict further modifications on the file.
(If placing the seal
.BR F_SEAL_WRITE ,
then it will be necessary to first unmap the shared writable mapping
-created in the previous step.)
+created in the previous step. Otherwise, behavior similar to
+.BR F_SEAL_WRITE
+can be achieved, by using
+.BR F_SEAL_FUTURE_WRITE
+which will prevent future writes via
+.BR mmap (2)
+and
+.BR write (2)
+from succeeding, while keeping existing shared writable mappings).
.IP 4.
A second process obtains a file descriptor for the
.BR tmpfs (5)
@@ -425,6 +433,7 @@ main(int argc, char *argv[])
fprintf(stderr, "\et\etg \- F_SEAL_GROW\en");
fprintf(stderr, "\et\ets \- F_SEAL_SHRINK\en");
fprintf(stderr, "\et\etw \- F_SEAL_WRITE\en");
+ fprintf(stderr, "\et\etW \- F_SEAL_FUTURE_WRITE\en");
fprintf(stderr, "\et\etS \- F_SEAL_SEAL\en");
exit(EXIT_FAILURE);
}
@@ -463,6 +472,8 @@ main(int argc, char *argv[])
seals |= F_SEAL_SHRINK;
if (strchr(seals_arg, \(aqw\(aq) != NULL)
seals |= F_SEAL_WRITE;
+ if (strchr(seals_arg, \(aqW\(aq) != NULL)
+ seals |= F_SEAL_FUTURE_WRITE;
if (strchr(seals_arg, \(aqS\(aq) != NULL)
seals |= F_SEAL_SEAL;
@@ -518,6 +529,8 @@ main(int argc, char *argv[])
printf(" GROW");
if (seals & F_SEAL_WRITE)
printf(" WRITE");
+ if (seals & F_SEAL_FUTURE_WRITE)
+ printf(" FUTURE_WRITE");
if (seals & F_SEAL_SHRINK)
printf(" SHRINK");
printf("\en");
--
2.21.0.360.g471c308f928-goog
Hello Joel,
On 3/14/19 10:48 PM, Joel Fernandes (Google) wrote:
> More details of the seal can be found in the LKML patch:
> https://lore.kernel.org/lkml/[email protected]/T/#t
>
> Signed-off-by: Joel Fernandes (Google) <[email protected]>
Thanks. Patch (finally) applied!
Cheers,
Michael
> ---
> man2/fcntl.2 | 15 +++++++++++++++
> 1 file changed, 15 insertions(+)
>
> diff --git a/man2/fcntl.2 b/man2/fcntl.2
> index fce4f4c2b3bd..e01e2c075b5b 100644
> --- a/man2/fcntl.2
> +++ b/man2/fcntl.2
> @@ -1525,6 +1525,21 @@ Furthermore, if there are any asynchronous I/O operations
> .RB ( io_submit (2))
> pending on the file,
> all outstanding writes will be discarded.
> +.TP
> +.BR F_SEAL_FUTURE_WRITE
> +If this seal is set, the contents of the file can be modified only from
> +existing writeable mappings that were created prior to the seal being set.
> +Any attempt to create a new writeable mapping on the memfd via
> +.BR mmap (2)
> +will fail with
> +.BR EPERM.
> +Also any attempts to write to the memfd via
> +.BR write (2)
> +will fail with
> +.BR EPERM.
> +This is useful in situations where existing writable mapped regions need to be
> +kept intact while preventing any future writes. For example, to share a
> +read-only memory buffer to other processes that only the sender can write to.
> .\"
> .SS File read/write hints
> Write lifetime hints can be used to inform the kernel about the relative
>
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
Hello Joel,
On 3/14/19 10:48 PM, Joel Fernandes (Google) wrote:
> More details of the seal can be found in the LKML patch:
> https://lore.kernel.org/lkml/[email protected]/T/#t
>
> Signed-off-by: Joel Fernandes (Google) <[email protected]>
Thanks! Patch applied
Cheers,
Michael
> ---
> man2/memfd_create.2 | 15 ++++++++++++++-
> 1 file changed, 14 insertions(+), 1 deletion(-)
>
> diff --git a/man2/memfd_create.2 b/man2/memfd_create.2
> index 15b1362f5525..3b7f032407ed 100644
> --- a/man2/memfd_create.2
> +++ b/man2/memfd_create.2
> @@ -280,7 +280,15 @@ in order to restrict further modifications on the file.
> (If placing the seal
> .BR F_SEAL_WRITE ,
> then it will be necessary to first unmap the shared writable mapping
> -created in the previous step.)
> +created in the previous step. Otherwise, behavior similar to
> +.BR F_SEAL_WRITE
> +can be achieved, by using
> +.BR F_SEAL_FUTURE_WRITE
> +which will prevent future writes via
> +.BR mmap (2)
> +and
> +.BR write (2)
> +from succeeding, while keeping existing shared writable mappings).
> .IP 4.
> A second process obtains a file descriptor for the
> .BR tmpfs (5)
> @@ -425,6 +433,7 @@ main(int argc, char *argv[])
> fprintf(stderr, "\et\etg \- F_SEAL_GROW\en");
> fprintf(stderr, "\et\ets \- F_SEAL_SHRINK\en");
> fprintf(stderr, "\et\etw \- F_SEAL_WRITE\en");
> + fprintf(stderr, "\et\etW \- F_SEAL_FUTURE_WRITE\en");
> fprintf(stderr, "\et\etS \- F_SEAL_SEAL\en");
> exit(EXIT_FAILURE);
> }
> @@ -463,6 +472,8 @@ main(int argc, char *argv[])
> seals |= F_SEAL_SHRINK;
> if (strchr(seals_arg, \(aqw\(aq) != NULL)
> seals |= F_SEAL_WRITE;
> + if (strchr(seals_arg, \(aqW\(aq) != NULL)
> + seals |= F_SEAL_FUTURE_WRITE;
> if (strchr(seals_arg, \(aqS\(aq) != NULL)
> seals |= F_SEAL_SEAL;
>
> @@ -518,6 +529,8 @@ main(int argc, char *argv[])
> printf(" GROW");
> if (seals & F_SEAL_WRITE)
> printf(" WRITE");
> + if (seals & F_SEAL_FUTURE_WRITE)
> + printf(" FUTURE_WRITE");
> if (seals & F_SEAL_SHRINK)
> printf(" SHRINK");
> printf("\en");
>
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/