[PATCH -manpage 0/2]

[PATCH -manpage 0/2]

[Joel Fernandes (Google)]

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

[PATCH -manpage 1/2] fcntl.2: Update manpage with new memfd F_SEAL_FUTURE_WRITE seal

[Joel Fernandes (Google)]

More details of the seal can be found in the LKML patch:
https://lore.kernel.org/lkml/20181120052137.74317-1-joel@joelfernandes.org/T/#t

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
 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

[PATCH -manpage 2/2] memfd_create.2: Update manpage with new memfd F_SEAL_FUTURE_WRITE seal

[Joel Fernandes (Google)]

More details of the seal can be found in the LKML patch:
https://lore.kernel.org/lkml/20181120052137.74317-1-joel@joelfernandes.org/T/#t

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
 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

Re: [PATCH -manpage 1/2] fcntl.2: Update manpage with new memfd F_SEAL_FUTURE_WRITE seal

[Michael Kerrisk (man-pages)]

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/20181120052137.74317-1-joel@joelfernandes.org/T/#t
>
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>

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/

Re: [PATCH -manpage 2/2] memfd_create.2: Update manpage with new memfd F_SEAL_FUTURE_WRITE seal

[Michael Kerrisk (man-pages)]

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/20181120052137.74317-1-joel@joelfernandes.org/T/#t
> 
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>

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/