Re: [PATCH v2 5/6] proc: use down_read_killable mmap_sem for /proc/pid/map_files

[Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>]

On Wed, 12 Jun 2019 16:14:28 -0700 Andrei Vagin <avagin@xxxxxxxxx> wrote:

> On Sun, Jun 09, 2019 at 01:09:00PM +0300, Konstantin Khlebnikov wrote:
> > Do not stuck forever if something wrong.
> > Killable lock allows to cleanup stuck tasks and simplifies investigation.
> 
> This patch breaks the CRIU project, because stat() returns EINTR instead
> of ENOENT:
> 
> [root@fc24 criu]# stat /proc/self/map_files/0-0
> stat: cannot stat '/proc/self/map_files/0-0': Interrupted system call
> 
> Here is one inline comment with the fix for this issue.
> 
> > @@ -2107,7 +2113,10 @@ static struct dentry *proc_map_files_lookup(struct inode *dir,
> >  	if (!mm)
> >  		goto out_put_task;
> >  
> > -	down_read(&mm->mmap_sem);
> > +	result = ERR_PTR(-EINTR);
> > +	if (down_read_killable(&mm->mmap_sem))
> > +		goto out_put_mm;
> > +
> 
> 	result = ERR_PTR(-ENOENT);
> 

yes, thanks.

--- a/fs/proc/base.c~proc-use-down_read_killable-mmap_sem-for-proc-pid-map_files-fix
+++ a/fs/proc/base.c
@@ -2117,6 +2117,7 @@ static struct dentry *proc_map_files_loo
 	if (down_read_killable(&mm->mmap_sem))
 		goto out_put_mm;
 
+	result = ERR_PTR(-ENOENT);
 	vma = find_exact_vma(mm, vm_start, vm_end);
 	if (!vma)
 		goto out_no_vma;



We started doing this trick of using

	ret = -EFOO;
	if (something)
		goto out;

decades ago because it generated slightly better code.  I rather doubt
whether that's still the case.

In fact this:

--- a/fs/proc/base.c~a
+++ a/fs/proc/base.c
@@ -2096,35 +2096,45 @@ static struct dentry *proc_map_files_loo
 	struct dentry *result;
 	struct mm_struct *mm;
 
-	result = ERR_PTR(-ENOENT);
 	task = get_proc_task(dir);
-	if (!task)
+	if (!task) {
+		result = ERR_PTR(-ENOENT);
 		goto out;
+	}
 
-	result = ERR_PTR(-EACCES);
-	if (!ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS))
+	if (!ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS)) {
+		result = ERR_PTR(-EACCES);
 		goto out_put_task;
+	}
 
-	result = ERR_PTR(-ENOENT);
-	if (dname_to_vma_addr(dentry, &vm_start, &vm_end))
+	if (dname_to_vma_addr(dentry, &vm_start, &vm_end)) {
+		result = ERR_PTR(-ENOENT);
 		goto out_put_task;
+	}
 
 	mm = get_task_mm(task);
-	if (!mm)
+	if (!mm) {
+		result = ERR_PTR(-ENOENT);
 		goto out_put_task;
+	}
 
-	result = ERR_PTR(-EINTR);
-	if (down_read_killable(&mm->mmap_sem))
+	if (down_read_killable(&mm->mmap_sem)) {
+		result = ERR_PTR(-EINTR);
 		goto out_put_mm;
+	}
 
-	result = ERR_PTR(-ENOENT);
 	vma = find_exact_vma(mm, vm_start, vm_end);
-	if (!vma)
+	if (!vma) {
+		result = ERR_PTR(-ENOENT);
 		goto out_no_vma;
+	}
 
-	if (vma->vm_file)
+	if (vma->vm_file) {
 		result = proc_map_files_instantiate(dentry, task,
 				(void *)(unsigned long)vma->vm_file->f_mode);
+	} else {
+		result = ERR_PTR(-ENOENT);
+	}
 
 out_no_vma:
 	up_read(&mm->mmap_sem);

makes no change to the generated assembly with gcc-7.2.0.

And I do think that the above style is clearer and more reliable, as
this bug demonstrates.