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SubjectRe: Oops in rpc_clnt_debugfs_register() from debugfs change
On Tue, Feb 12, 2019 at 03:42:14PM +0100, Greg Kroah-Hartman wrote:
> On Tue, Feb 12, 2019 at 03:37:20PM +0100, Greg Kroah-Hartman wrote:
> > On Tue, Feb 12, 2019 at 02:31:14PM +0000, David Howells wrote:
> > > I've bisected an oops that occurs in rpc_clnt_debugfs_register() trying to
> > > dereference a pointer with -EACCES in it. This is the causing commit, though
> > > I suspect the bug is in sunrpc expecting to see NULL rather than an error.
> > >
> > > ff9fb72bc07705c00795ca48631f7fffe24d2c6b is the first bad commit
> > > commit ff9fb72bc07705c00795ca48631f7fffe24d2c6b
> > > Author: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > > Date: Wed Jan 23 11:28:14 2019 +0100
> > >
> > > debugfs: return error values, not NULL
> > >
> > > When an error happens, debugfs should return an error pointer value, not
> > > NULL. This will prevent the totally theoretical error where a debugfs
> > > call fails due to lack of memory, returning NULL, and that dentry value
> > > is then passed to another debugfs call, which would end up succeeding,
> > > creating a file at the root of the debugfs tree, but would then be
> > > impossible to remove (because you can not remove the directory NULL).
> > >
> > > So, to make everyone happy, always return errors, this makes the users
> > > of debugfs much simpler (they do not have to ever check the return
> > > value), and everyone can rest easy.
> > > ...
> > >
> > > The attached oops occurs during boot from the gssproxy process in
> > > rpc_clnt_debugfs_register(). The code at this point is:
> > >
> > > 0xffffffff8195cbdd <+450>: mov 0x50(%rax),%rcx <--- oopsing
> > > 0xffffffff8195cbe1 <+454>: mov $0xffffffff821cc8ba,%rdx
> > > 0xffffffff8195cbe8 <+461>: mov $0x18,%esi
> > > 0xffffffff8195cbed <+466>: lea -0x30(%rbp),%rdi
> > > 0xffffffff8195cbf1 <+470>: callq 0xffffffff819db773 <snprintf>
> > >
> > > RAX is -EACCES.
> > >
> > > Looking in the source:
> > >
> > > len = snprintf(name, sizeof(name), "../../rpc_xprt/%s",
> > > xprt->debugfs->d_name.name);
> > >
> > > I think xprt->debugfs is the value in RAX.
> > >
> > > (gdb) p &((struct dentry *)0)->d_name.name
> > > $5 = (const unsigned char **) 0x50 <irq_stack_union+80>
> > >
> > > which matches the offset on the oopsing MOV instruction.
> > >
> > > This is with linus/master (aa0c38cf39de73bf7360a3da8f1707601261e518).
> >
> > Ugh, yeah, I see the problem, sorry about that.
> >
> > I wonder why the debugfs call is always failing, that's not good...
> >
> > let me dig and see if I already have a patch for this...
>
> I have a much larger cleanup patch for this code, but this single line
> change should solve the issue for now. Can you test it to verify?
>
> thanks,
>
> greg k-h
>
> ------------------
>
> diff --git a/net/sunrpc/debugfs.c b/net/sunrpc/debugfs.c
> index 45a033329cd4..19bb356230ed 100644
> --- a/net/sunrpc/debugfs.c
> +++ b/net/sunrpc/debugfs.c
> @@ -146,7 +146,7 @@ rpc_clnt_debugfs_register(struct rpc_clnt *clnt)
> rcu_read_lock();
> xprt = rcu_dereference(clnt->cl_xprt);
> /* no "debugfs" dentry? Don't bother with the symlink. */
> - if (!xprt->debugfs) {
> + if (IS_ERR_OR_NULL(xprt->debugfs)) {
> rcu_read_unlock();
> return;
> }


And, if you want my larger fix that I will be sending to netdev one of
these days, here's that one. It includes the above patch as part of it.

thanks,

greg k-h

---------------

commit 8d885c486153d1731c14a6a435774a4e9ccd1ebc
Author: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Date: Fri Jan 4 13:40:56 2019 +0100

sunrpc: fix changelog

diff --git a/net/sunrpc/debugfs.c b/net/sunrpc/debugfs.c
index 45a033329cd4..ca63f6ed873f 100644
--- a/net/sunrpc/debugfs.c
+++ b/net/sunrpc/debugfs.c
@@ -135,18 +135,15 @@ rpc_clnt_debugfs_register(struct rpc_clnt *clnt)

/* make the per-client dir */
clnt->cl_debugfs = debugfs_create_dir(name, rpc_clnt_dir);
- if (!clnt->cl_debugfs)
- return;

/* make tasks file */
- if (!debugfs_create_file("tasks", S_IFREG | 0400, clnt->cl_debugfs,
- clnt, &tasks_fops))
- goto out_err;
+ debugfs_create_file("tasks", S_IFREG | 0400, clnt->cl_debugfs, clnt,
+ &tasks_fops);

rcu_read_lock();
xprt = rcu_dereference(clnt->cl_xprt);
/* no "debugfs" dentry? Don't bother with the symlink. */
- if (!xprt->debugfs) {
+ if (IS_ERR_OR_NULL(xprt->debugfs)) {
rcu_read_unlock();
return;
}
@@ -157,8 +154,7 @@ rpc_clnt_debugfs_register(struct rpc_clnt *clnt)
if (len >= sizeof(name))
goto out_err;

- if (!debugfs_create_symlink("xprt", clnt->cl_debugfs, name))
- goto out_err;
+ debugfs_create_symlink("xprt", clnt->cl_debugfs, name);

return;
out_err:
@@ -237,15 +233,10 @@ rpc_xprt_debugfs_register(struct rpc_xprt *xprt)

/* make the per-client dir */
xprt->debugfs = debugfs_create_dir(name, rpc_xprt_dir);
- if (!xprt->debugfs)
- return;

/* make tasks file */
- if (!debugfs_create_file("info", S_IFREG | 0400, xprt->debugfs,
- xprt, &xprt_info_fops)) {
- debugfs_remove_recursive(xprt->debugfs);
- xprt->debugfs = NULL;
- }
+ debugfs_create_file("info", S_IFREG | 0400, xprt->debugfs, xprt,
+ &xprt_info_fops);

atomic_set(&xprt->inject_disconnect, rpc_inject_disconnect);
}
@@ -308,22 +299,6 @@ static const struct file_operations fault_disconnect_fops = {
.release = fault_release,
};

-static struct dentry *
-inject_fault_dir(struct dentry *topdir)
-{
- struct dentry *faultdir;
-
- faultdir = debugfs_create_dir("inject_fault", topdir);
- if (!faultdir)
- return NULL;
-
- if (!debugfs_create_file("disconnect", S_IFREG | 0400, faultdir,
- NULL, &fault_disconnect_fops))
- return NULL;
-
- return faultdir;
-}
-
void __exit
sunrpc_debugfs_exit(void)
{
@@ -338,25 +313,13 @@ void __init
sunrpc_debugfs_init(void)
{
topdir = debugfs_create_dir("sunrpc", NULL);
- if (!topdir)
- return;
-
- rpc_fault_dir = inject_fault_dir(topdir);
- if (!rpc_fault_dir)
- goto out_remove;

rpc_clnt_dir = debugfs_create_dir("rpc_clnt", topdir);
- if (!rpc_clnt_dir)
- goto out_remove;

rpc_xprt_dir = debugfs_create_dir("rpc_xprt", topdir);
- if (!rpc_xprt_dir)
- goto out_remove;

- return;
-out_remove:
- debugfs_remove_recursive(topdir);
- topdir = NULL;
- rpc_fault_dir = NULL;
- rpc_clnt_dir = NULL;
+ rpc_fault_dir = debugfs_create_dir("inject_fault", topdir);
+
+ debugfs_create_file("disconnect", S_IFREG | 0400, rpc_fault_dir, NULL,
+ &fault_disconnect_fops);
}
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