On 08/04/2017 12:07 PM, riel@xxxxxxxxxx wrote: > From: Rik van Riel <riel@xxxxxxxxxx> > > 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. > > 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. > > This is similar to the OpenBSD minherit syscall with MAP_INHERIT_ZERO: > > https://man.openbsd.org/minherit.2 > > Reported-by: Florian Weimer <fweimer@xxxxxxxxxx> > Reported-by: Colm MacCártaigh <colm@xxxxxxxxxxxx> > Signed-off-by: Rik van Riel <riel@xxxxxxxxxx> > --- > arch/alpha/include/uapi/asm/mman.h | 3 +++ > arch/mips/include/uapi/asm/mman.h | 3 +++ > arch/parisc/include/uapi/asm/mman.h | 3 +++ > arch/xtensa/include/uapi/asm/mman.h | 3 +++ > fs/proc/task_mmu.c | 1 + > include/linux/mm.h | 2 +- > include/uapi/asm-generic/mman-common.h | 3 +++ > kernel/fork.c | 8 ++++++-- > mm/madvise.c | 8 ++++++++ > mm/memory.c | 10 ++++++++++ > 10 files changed, 41 insertions(+), 3 deletions(-) > This didn't seem 'quite right' to me for shared mappings and/or file backed mappings. I wasn't exactly sure what it 'should' do in such cases. So, I tried it with a mapping created as follows: addr = mmap(ADDR, page_size, PROT_READ | PROT_WRITE, MAP_ANONYMOUS|MAP_SHARED, -1, 0); When setting MADV_WIPEONFORK on the vma/mapping, I got the following at task exit time: [ 694.558290] ------------[ cut here ]------------ [ 694.558978] kernel BUG at mm/filemap.c:212! [ 694.559476] invalid opcode: 0000 [#1] SMP [ 694.560023] Modules linked in: ip6t_REJECT nf_reject_ipv6 ip6t_rpfilter xt_conntrack ebtable_broute bridge stp llc ebtable_nat ip6table_nat nf_conntrack_ipv6 nf_defrag_ipv6 nf_nat_ipv6 ip6table_raw ip6table_mangle ip6table_security iptable_nat nf_conntrack_ipv4 nf_defrag_ipv4 nf_nat_ipv4 nf_nat nf_conntrack iptable_raw iptable_mangle 9p iptable_security ebtable_filter ebtables ip6table_filter ip6_tables snd_hda_codec_generic snd_hda_intel snd_hda_codec snd_hwdep snd_hda_core snd_seq ppdev snd_seq_device joydev crct10dif_pclmul crc32_pclmul crc32c_intel snd_pcm ghash_clmulni_intel 9pnet_virtio virtio_balloon snd_timer 9pnet parport_pc snd parport i2c_piix4 soundcore nfsd auth_rpcgss nfs_acl lockd grace sunrpc virtio_net virtio_blk virtio_console 8139too qxl drm_kms_helper ttm drm serio_raw 8139cp [ 694.571554] mii virtio_pci ata_generic virtio_ring virtio pata_acpi [ 694.572608] CPU: 3 PID: 1200 Comm: test_wipe2 Not tainted 4.13.0-rc3+ #8 [ 694.573778] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.9.1-1.fc24 04/01/2014 [ 694.574917] task: ffff880137178040 task.stack: ffffc900019d4000 [ 694.575650] RIP: 0010:__delete_from_page_cache+0x344/0x410 [ 694.576409] RSP: 0018:ffffc900019d7a88 EFLAGS: 00010082 [ 694.577238] RAX: 0000000000000021 RBX: ffffea00047d0e00 RCX: 0000000000000006 [ 694.578537] RDX: 0000000000000000 RSI: 0000000000000096 RDI: ffff88023fd0db90 [ 694.579774] RBP: ffffc900019d7ad8 R08: 00000000000882b6 R09: 000000000000028a [ 694.580754] R10: ffffc900019d7da8 R11: ffffffff8211184d R12: ffffea00047d0e00 [ 694.582040] R13: 0000000000000000 R14: 0000000000000202 R15: ffff8801384439e8 [ 694.583236] FS: 0000000000000000(0000) GS:ffff88023fd00000(0000) knlGS:0000000000000000 [ 694.584607] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [ 694.585409] CR2: 00007ff77a8da618 CR3: 0000000001e09000 CR4: 00000000001406e0 [ 694.586547] Call Trace: [ 694.586996] delete_from_page_cache+0x54/0x110 [ 694.587481] truncate_inode_page+0xab/0x120 [ 694.588110] shmem_undo_range+0x498/0xa50 [ 694.588813] ? save_stack_trace+0x1b/0x20 [ 694.589529] ? set_track+0x70/0x140 [ 694.590150] ? init_object+0x69/0xa0 [ 694.590722] ? __inode_wait_for_writeback+0x73/0xe0 [ 694.591525] shmem_truncate_range+0x16/0x40 [ 694.592268] shmem_evict_inode+0xb1/0x190 [ 694.592735] evict+0xbb/0x1c0 [ 694.593147] iput+0x1c0/0x210 [ 694.593497] dentry_unlink_inode+0xb4/0x150 [ 694.593982] __dentry_kill+0xc1/0x150 [ 694.594400] dput+0x1c8/0x1e0 [ 694.594745] __fput+0x172/0x1e0 [ 694.595103] ____fput+0xe/0x10 [ 694.595463] task_work_run+0x80/0xa0 [ 694.595886] do_exit+0x2d6/0xb50 [ 694.596323] ? __do_page_fault+0x288/0x4a0 [ 694.596818] do_group_exit+0x47/0xb0 [ 694.597249] SyS_exit_group+0x14/0x20 [ 694.597682] entry_SYSCALL_64_fastpath+0x1a/0xa5 [ 694.598198] RIP: 0033:0x7ff77a5e78c8 [ 694.598612] RSP: 002b:00007ffc5aece318 EFLAGS: 00000246 ORIG_RAX: 00000000000000e7 [ 694.599804] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007ff77a5e78c8 [ 694.600609] RDX: 0000000000000000 RSI: 000000000000003c RDI: 0000000000000000 [ 694.601424] RBP: 00007ff77a8da618 R08: 00000000000000e7 R09: ffffffffffffff98 [ 694.602224] R10: 0000000000000003 R11: 0000000000000246 R12: 0000000000000001 [ 694.603151] R13: 00007ff77a8dbc60 R14: 0000000000000000 R15: 0000000000000000 [ 694.603984] Code: 60 f3 c5 81 e8 2e 7e 03 00 0f 0b 48 c7 c6 60 f3 c5 81 4c 89 e7 e8 1d 7e 03 00 0f 0b 48 c7 c6 00 f4 c5 81 4c 89 e7 e8 0c 7e 03 00 <0f> 0b 48 c7 c6 38 f3 c5 81 4c 89 e7 e8 fb 7d 03 00 0f 0b 48 c7 [ 694.606500] RIP: __delete_from_page_cache+0x344/0x410 RSP: ffffc900019d7a88 [ 694.607426] ---[ end trace 55e6b04ae95d8ce3 ]--- BTW, this was on 4.13.0-rc3 + your patches. Simple test program is below. -- Mike Kravetz #include <unistd.h> #include <stdlib.h> #include <stdio.h> #include <sys/mman.h> #include <errno.h> #define MADV_WIPEONFORK 18 #define ADDR (void *)(0x0UL) int main(int argc, char ** argv) { unsigned long page_size; int ret; void *addr; char foo; page_size = sysconf(_SC_PAGE_SIZE); addr = mmap(ADDR, page_size, PROT_READ | PROT_WRITE, MAP_ANONYMOUS|MAP_SHARED, -1, 0); if (addr == MAP_FAILED) { perror("mmap"); exit (1); } printf("Parent writing 'a' to page\n"); *((char *)addr) = 'a'; ret = madvise(addr, page_size, MADV_WIPEONFORK); if (ret) { perror("madvise"); exit (1); } if (fork()) { /* In parent */ sleep(1); } else { /* In child */ foo = *((char *)addr); printf("child read '%c' from page\n", foo); } return ret; } -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
