path: root/arch/xtensa
diff options
authorRik van Riel <>2017-09-06 16:25:15 -0700
committerLinus Torvalds <>2017-09-06 17:27:30 -0700
commitd2cd9ede6e193dd7d88b6d27399e96229a551b19 (patch)
tree8d0b95baf0ecfba3d2361866abf2b6f614b7f0d8 /arch/xtensa
parentdf3735c5b40fad8d0d28eb8ab065fe955b3347ee (diff)
mm,fork: introduce MADV_WIPEONFORK
Introduce MADV_WIPEONFORK semantics, which result in a VMA being empty in the child process after fork. This differs from MADV_DONTFORK in one important way. If a child process accesses memory that was MADV_WIPEONFORK, it will get zeroes. The address ranges are still valid, they are just empty. If a child process accesses memory that was MADV_DONTFORK, it will get a segmentation fault, since those address ranges are no longer valid in the child after fork. Since MADV_DONTFORK also seems to be used to allow very large programs to fork in systems with strict memory overcommit restrictions, changing the semantics of MADV_DONTFORK might break existing programs. MADV_WIPEONFORK only works on private, anonymous VMAs. The use case is libraries that store or cache information, and want to know that they need to regenerate it in the child process after fork. Examples of this would be: - systemd/pulseaudio API checks (fail after fork) (replacing a getpid check, which is too slow without a PID cache) - PKCS#11 API reinitialization check (mandated by specification) - glibc's upcoming PRNG (reseed after fork) - OpenSSL PRNG (reseed after fork) The security benefits of a forking server having a re-inialized PRNG in every child process are pretty obvious. However, due to libraries having all kinds of internal state, and programs getting compiled with many different versions of each library, it is unreasonable to expect calling programs to re-initialize everything manually after fork. A further complication is the proliferation of clone flags, programs bypassing glibc's functions to call clone directly, and programs calling unshare, causing the glibc pthread_atfork hook to not get called. It would be better to have the kernel take care of this automatically. The patch also adds MADV_KEEPONFORK, to undo the effects of a prior MADV_WIPEONFORK. This is similar to the OpenBSD minherit syscall with MAP_INHERIT_ZERO: [ numerically order arch/parisc/include/uapi/asm/mman.h #defines] Link: Signed-off-by: Rik van Riel <> Reported-by: Florian Weimer <> Reported-by: Colm MacCártaigh <> Reviewed-by: Mike Kravetz <> Cc: "H. Peter Anvin" <> Cc: "Kirill A. Shutemov" <> Cc: Andy Lutomirski <> Cc: Dave Hansen <> Cc: Ingo Molnar <> Cc: Helge Deller <> Cc: Kees Cook <> Cc: Matthew Wilcox <> Cc: Thomas Gleixner <> Cc: Will Drewry <> Cc: <> Signed-off-by: Andrew Morton <> Signed-off-by: Linus Torvalds <>
Diffstat (limited to 'arch/xtensa')
1 files changed, 3 insertions, 0 deletions
diff --git a/arch/xtensa/include/uapi/asm/mman.h b/arch/xtensa/include/uapi/asm/mman.h
index 8ce77a2e9bab..b15b278aa314 100644
--- a/arch/xtensa/include/uapi/asm/mman.h
+++ b/arch/xtensa/include/uapi/asm/mman.h
@@ -103,6 +103,9 @@
overrides the coredump filter bits */
#define MADV_DODUMP 17 /* Clear the MADV_NODUMP flag */
+#define MADV_WIPEONFORK 18 /* Zero memory on fork, child only */
/* compatibility flags */
#define MAP_FILE 0