tracing/uprobe: Add support for overlayfs

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'

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.

Link: http://lkml.kernel.org/r/[email protected]

Reviewed-by: Josef Bacik <[email protected]>
Reviewed-by: Masami Hiramatsu <[email protected]>
Reviewed-by: Srikar Dronamraju <[email protected]>
Signed-off-by: Howard McLauchlan <[email protected]>
Signed-off-by: Steven Rostedt (VMware) <[email protected]>

Author: Howard McLauchlan

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)) {