[PATCH v2 2/7] kernel/fork: factor out replacing the current MM exe_file

David Hildenbrand
2021-08-16T19:50:09Z

Let's factor the main logic out into replace_mm_exe_file(), such that
all mm->exe_file logic is contained in kernel/fork.c.

While at it, perform some simple cleanups that are possible now that
we're simplifying the individual functions.

Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
include/linux/mm.h | 1 +
kernel/fork.c | 44 +++++++++++++++++++++++++++++++++++++++++---
kernel/sys.c | 33 +--------------------------------
3 files changed, 43 insertions(+), 35 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 7ca22e6e694a..48c6fa9ab792 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2581,6 +2581,7 @@ extern int mm_take_all_locks(struct mm_struct *mm);
extern void mm_drop_all_locks(struct mm_struct *mm);

extern void set_mm_exe_file(struct mm_struct *mm, struct file *new_exe_file);
+extern int replace_mm_exe_file(struct mm_struct *mm, struct file *new_exe_file);
extern struct file *get_mm_exe_file(struct mm_struct *mm);
extern struct file *get_task_exe_file(struct task_struct *task);

diff --git a/kernel/fork.c b/kernel/fork.c
index bc94b2cc5995..eedce5c77041 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1148,9 +1148,7 @@ void mmput_async(struct mm_struct *mm)
*
* Main users are mmput() and sys_execve(). Callers prevent concurrent
* invocations: in mmput() nobody alive left, in execve task is single
- * threaded. sys_prctl(PR_SET_MM_MAP/EXE_FILE) also needs to set the
- * mm->exe_file, but does so without using set_mm_exe_file() in order
- * to avoid the need for any locks.
+ * threaded.
*/
void set_mm_exe_file(struct mm_struct *mm, struct file *new_exe_file)
{
@@ -1170,6 +1168,46 @@ void set_mm_exe_file(struct mm_struct *mm, struct file *new_exe_file)
fput(old_exe_file);
}

+/**
+ * replace_mm_exe_file - replace a reference to the mm's executable file
+ *
+ * This changes mm's executable file (shown as symlink /proc/[pid]/exe),
+ * dealing with concurrent invocation and without grabbing the mmap lock in
+ * write mode.
+ *
+ * Main user is sys_prctl(PR_SET_MM_MAP/EXE_FILE).
+ */
+int replace_mm_exe_file(struct mm_struct *mm, struct file *new_exe_file)
+{
+ struct vm_area_struct *vma;
+ struct file *old_exe_file;
+ int ret = 0;
+
+ /* Forbid mm->exe_file change if old file still mapped. */
+ old_exe_file = get_mm_exe_file(mm);
+ if (old_exe_file) {
+ mmap_read_lock(mm);
+ for (vma = mm->mmap; vma && !ret; vma = vma->vm_next) {
+ if (!vma->vm_file)
+ continue;
+ if (path_equal(&vma->vm_file->f_path,
+ &old_exe_file->f_path))
+ ret = -EBUSY;
+ }
+ mmap_read_unlock(mm);
+ fput(old_exe_file);
+ if (ret)
+ return ret;
+ }
+
+ /* set the new file, lockless */
+ get_file(new_exe_file);
+ old_exe_file = xchg(&mm->exe_file, new_exe_file);
+ if (old_exe_file)
+ fput(old_exe_file);
+ return 0;
+}
+
/**
* get_mm_exe_file - acquire a reference to the mm's executable file
*
diff --git a/kernel/sys.c b/kernel/sys.c
index ef1a78f5d71c..30c12e54585a 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -1846,7 +1846,6 @@ SYSCALL_DEFINE1(umask, int, mask)
static int prctl_set_mm_exe_file(struct mm_struct *mm, unsigned int fd)
{
struct fd exe;
- struct file *old_exe, *exe_file;
struct inode *inode;
int err;

@@ -1869,40 +1868,10 @@ static int prctl_set_mm_exe_file(struct mm_struct *mm, unsigned int fd)
if (err)
goto exit;

- /*
- * Forbid mm->exe_file change if old file still mapped.
- */
- exe_file = get_mm_exe_file(mm);
- err = -EBUSY;
- if (exe_file) {
- struct vm_area_struct *vma;
-
- mmap_read_lock(mm);
- for (vma = mm->mmap; vma; vma = vma->vm_next) {
- if (!vma->vm_file)
- continue;
- if (path_equal(&vma->vm_file->f_path,
- &exe_file->f_path))
- goto exit_err;
- }
-
- mmap_read_unlock(mm);
- fput(exe_file);
- }
-
- err = 0;
- /* set the new file, lockless */
- get_file(exe.file);
- old_exe = xchg(&mm->exe_file, exe.file);
- if (old_exe)
- fput(old_exe);
+ err = replace_mm_exe_file(mm, exe.file);
exit:
fdput(exe);
return err;
-exit_err:
- mmap_read_unlock(mm);
- fput(exe_file);
- goto exit;
}

/*
--
2.31.1