Re: [PATCH] uprobe: add support for overlayfs

[Srikar Dronamraju]

* Steven Rostedt <rostedt@xxxxxxxxxxx> [2018-04-10 20:00:59]:

> On Tue, 10 Apr 2018 16:10:30 -0700
> Howard McLauchlan <hmclauchlan@xxxxxx> wrote:
> 
> > uprobes cannot successfully attach to binaries located in a directory
> > mounted with overlayfs.
> > 
> > To verify, create directories for mounting overlayfs
> > (upper,lower,work,merge), move some binary into merge/ and use readelf
> > to obtain some known instruction of the binary. I used /bin/true and the
> > entry instruction(0x13b0):
> > 
> > 	$ mount -t overlay overlay -o lowerdir=lower,upperdir=upper,workdir=work merge
> > 	$ cd /sys/kernel/debug/tracing
> > 	$ echo 'p:true_entry PATH_TO_MERGE/merge/true:0x13b0' > uprobe_events
> > 	$ echo 1 > events/uprobes/true_entry/enable
> > 
> > This returns 'bash: echo: write error: Input/output error' and dmesg
> > tells us 'event trace: Could not enable event true_entry'
> 
> I tried the above and verified that it is a problem and your patch
> solves it.
> 
> Thanks, I'll apply it and start my tests on it.
> 
> -- Steve
> 
> > 
> > This change makes create_trace_uprobe() look for the real inode of a
> > dentry. In the case of normal filesystems, this simplifies to just
> > returning the inode. In the case of overlayfs(and similar fs) we will
> > obtain the underlying dentry and corresponding inode, upon which uprobes
> > can successfully register.
> > 
> > Running the example above with the patch applied, we can see that the
> > uprobe is enabled and will output to trace as expected.
> > 


Looks good to me.

Reviewed-by: Srikar Dronamraju <srikar@xxxxxxxxxxxxxxxxxx>

> > Reviewed-by: Josef Bacik <jbacik@xxxxxx>
> > Signed-off-by: Howard McLauchlan <hmclauchlan@xxxxxx>
> > ---
> >  kernel/trace/trace_uprobe.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/kernel/trace/trace_uprobe.c b/kernel/trace/trace_uprobe.c
> > index 2014f4351ae0..17c65fa4136d 100644
> > --- a/kernel/trace/trace_uprobe.c
> > +++ b/kernel/trace/trace_uprobe.c
> > @@ -446,7 +446,7 @@ static int create_trace_uprobe(int argc, char **argv)
> >  	if (ret)
> >  		goto fail_address_parse;
> >  
> > -	inode = igrab(d_inode(path.dentry));
> > +	inode = igrab(d_real_inode(path.dentry));
> >  	path_put(&path);
> >  
> >  	if (!inode || !S_ISREG(inode->i_mode)) {
>