More details of the seal can be found in the LKML patch: https://lore.kernel.org/lkml/20181120052137.74317-1-joel@joelfernandes.org/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 03533d65b49d..54772f94964c 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
More details of the seal can be found in the LKML patch: https://lore.kernel.org/lkml/20181120052137.74317-1-joel@joelfernandes.org/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 3cd392d1b4d9..fce2bf8d0fff 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, "\t\tg - F_SEAL_GROW\n"); fprintf(stderr, "\t\ts - F_SEAL_SHRINK\n"); fprintf(stderr, "\t\tw - F_SEAL_WRITE\n"); + fprintf(stderr, "\t\tW - F_SEAL_FUTURE_WRITE\n"); fprintf(stderr, "\t\tS - F_SEAL_SEAL\n"); 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("\n");
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Joel Fernandes (Google)