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Subject[PATCH net-next 1/4] mm: provide a mmap_hook infrastructure
Date
When adding tcp mmap() implementation, I forgot that socket lock
had to be taken before current->mm->mmap_sem. syzbot eventually caught
the bug.

This patch provides a new mmap_hook() method in struct file_operations
that might be provided by fs to implement a finer control of whats
to be done before and after do_mmap_pgoff() and/or the mm->mmap_sem
acquire/release.

This is used in following patches by networking and TCP stacks
to solve the lockdep issue, and also allows some preparation
and cleanup work being done before/after mmap_sem is held,
allowing better scalability in multi-threading programs.

Fixes: 93ab6cc69162 ("tcp: implement mmap() for zero copy receive")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
---
include/linux/fs.h | 6 ++++++
mm/util.c | 19 ++++++++++++++++++-
2 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 92efaf1f89775f7b017477617dd983c10e0dc4d2..ef3526f84686585678861fc585efea974a69ca55 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1698,6 +1698,11 @@ struct block_device_operations;
#define NOMMU_VMFLAGS \
(NOMMU_MAP_READ | NOMMU_MAP_WRITE | NOMMU_MAP_EXEC)

+enum mmap_hook {
+ MMAP_HOOK_PREPARE,
+ MMAP_HOOK_ROLLBACK,
+ MMAP_HOOK_COMMIT,
+};

struct iov_iter;

@@ -1714,6 +1719,7 @@ struct file_operations {
long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
int (*mmap) (struct file *, struct vm_area_struct *);
+ int (*mmap_hook) (struct file *, enum mmap_hook);
unsigned long mmap_supported_flags;
int (*open) (struct inode *, struct file *);
int (*flush) (struct file *, fl_owner_t id);
diff --git a/mm/util.c b/mm/util.c
index 1fc4fa7576f762bbbf341f056ca6d0be803a423f..3ddb18ab367f069d5884083e992e999546ccd995 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -350,11 +350,28 @@ unsigned long vm_mmap_pgoff(struct file *file, unsigned long addr,

ret = security_mmap_file(file, prot, flag);
if (!ret) {
- if (down_write_killable(&mm->mmap_sem))
+ int (*mmap_hook)(struct file *, enum mmap_hook) = NULL;
+
+ if (file) {
+ mmap_hook = file->f_op->mmap_hook;
+
+ if (mmap_hook) {
+ ret = mmap_hook(file, MMAP_HOOK_PREPARE);
+ if (ret)
+ return ret;
+ }
+ }
+ if (down_write_killable(&mm->mmap_sem)) {
+ if (mmap_hook)
+ mmap_hook(file, MMAP_HOOK_ROLLBACK);
return -EINTR;
+ }
ret = do_mmap_pgoff(file, addr, len, prot, flag, pgoff,
&populate, &uf);
up_write(&mm->mmap_sem);
+ if (mmap_hook)
+ mmap_hook(file, IS_ERR(ret) ? MMAP_HOOK_ROLLBACK :
+ MMAP_HOOK_COMMIT);
userfaultfd_unmap_complete(mm, &uf);
if (populate)
mm_populate(ret, populate);
--
2.17.0.484.g0c8726318c-goog
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