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

[Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>]

On 13.06.2019 2:14, Andrei Vagin 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

Good catch.

It seems CRIU tests has good coverage for darkest corners of kernel API.
Kernel CI projects should use it. I suppose you know how to promote this. =)

> Here is one inline comment with the fix for this issue.
>
> > It seems ->d_revalidate() could return any error (except ECHILD) to
> > abort validation and pass error as result of lookup sequence.
> >
> > Signed-off-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
> > Reviewed-by: Roman Gushchin <guro@xxxxxx>
> > Reviewed-by: Cyrill Gorcunov <gorcunov@xxxxxxxxx>
> > Reviewed-by: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
>
> It was nice to see all four of you in one place :).
>
> > Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> > ---
> >   fs/proc/base.c |   27 +++++++++++++++++++++------
> >   1 file changed, 21 insertions(+), 6 deletions(-)
> >
> > diff --git a/fs/proc/base.c b/fs/proc/base.c
> > index 9c8ca6cd3ce4..515ab29c2adf 100644
> > --- a/fs/proc/base.c
> > +++ b/fs/proc/base.c
> > @@ -1962,9 +1962,12 @@ static int map_files_d_revalidate(struct dentry *dentry, unsigned int flags)
> >   		goto out;
> >   
> >   	if (!dname_to_vma_addr(dentry, &vm_start, &vm_end)) {
> > -		down_read(&mm->mmap_sem);
> > -		exact_vma_exists = !!find_exact_vma(mm, vm_start, vm_end);
> > -		up_read(&mm->mmap_sem);
> > +		status = down_read_killable(&mm->mmap_sem);
> > +		if (!status) {
> > +			exact_vma_exists = !!find_exact_vma(mm, vm_start,
> > +							    vm_end);
> > +			up_read(&mm->mmap_sem);
> > +		}
> >   	}
> >   
> >   	mmput(mm);
> > @@ -2010,8 +2013,11 @@ static int map_files_get_link(struct dentry *dentry, struct path *path)
> >   	if (rc)
> >   		goto out_mmput;
> >   
> > +	rc = down_read_killable(&mm->mmap_sem);
> > +	if (rc)
> > +		goto out_mmput;
> > +
> >   	rc = -ENOENT;
> > -	down_read(&mm->mmap_sem);
> >   	vma = find_exact_vma(mm, vm_start, vm_end);
> >   	if (vma && vma->vm_file) {
> >   		*path = vma->vm_file->f_path;
> > @@ -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);
>
> >   	vma = find_exact_vma(mm, vm_start, vm_end);
> >   	if (!vma)
> >   		goto out_no_vma;
> > @@ -2118,6 +2127,7 @@ static struct dentry *proc_map_files_lookup(struct inode *dir,
> >   
> >   out_no_vma:
> >   	up_read(&mm->mmap_sem);
> > +out_put_mm:
> >   	mmput(mm);
> >   out_put_task:
> >   	put_task_struct(task);
> > @@ -2160,7 +2170,12 @@ proc_map_files_readdir(struct file *file, struct dir_context *ctx)
> >   	mm = get_task_mm(task);
> >   	if (!mm)
> >   		goto out_put_task;
> > -	down_read(&mm->mmap_sem);
> > +
> > +	ret = down_read_killable(&mm->mmap_sem);
> > +	if (ret) {
> > +		mmput(mm);
> > +		goto out_put_task;
> > +	}
> >   
> >   	nr_files = 0;
> >