Re: build_path_from_dentry_optional_prefix() may schedule from invalid context

[Pavel Shilovsky]

ÑÐ, 21 ÑÐÐÑ. 2019 Ð. Ð 15:38, Al Viro <viro@xxxxxxxxxxxxxxxxxx>:
>
> On Thu, Sep 19, 2019 at 05:11:54PM -0700, Pavel Shilovsky wrote:
>
> > Good catch. I think we should have another version of
> > build_path_from_dentry() which takes pre-allocated (probably on stack)
> > full_path as an argument. This would allow us to avoid allocations
> > under the spin lock.
>
> On _stack_?  For relative pathname?  Er...  You do realize that
> kernel stack is small, right?  And said relative pathname can
> bloody well be up to 4Kb (i.e. the half of said stack already,
> on top of whatever the call chain has already eaten up)...

My idea was to use a small stack-allocated array which satisfies most
cases (say 100-200 bytes) and fallback to dynamic a heap allocation
for longer path names.

>
> BTW, looking at build_path_from_dentry()...  WTF is this?
>                 temp = temp->d_parent;
>                 if (temp == NULL) {
>                         cifs_dbg(VFS, "corrupt dentry\n");
>                         rcu_read_unlock();
>                         return NULL;
>                 }
> Why not check for any number of other forms of memory corruption?
> Like, say it, if (temp == (void *)0xf0adf0adf0adf0ad)?
>
> IOW, kindly lose that nonsense.  More importantly, why bother
> with that kmalloc()?  Just __getname() in the very beginning
> and __putname() on failure (and for freeing the result afterwards).
>
> What's more, you are open-coding dentry_path_raw(), badly.
> The only differences are
>         * use of dirsep instead of '/' and
>         * a prefix slapped in the beginning.
>
> I'm fairly sure that
>         char *buf = __getname();
>         char *s;
>
>         *to_free = NULL;
>         if (unlikely(!buf))
>                 return NULL;
>
>         s = dentry_path_raw(dentry, buf, PATH_MAX);
>         if (IS_ERR(s) || s < buf + prefix_len)
>                 __putname(buf);
>                 return NULL; // assuming that you don't care about details
>         }
>
>         if (dirsep != '/') {
>                 char *p = s;
>                 while ((p = strchr(p, '/')) != NULL)
>                         *p++ = dirsep;
>         }
>
>         s -= prefix_len;
>         memcpy(s, prefix, prefix_len);
>
>         *to_free = buf;
>         return s;
>
> would end up being faster, not to mention much easier to understand.
> With the caller expected to pass &to_free among the arguments and
> __putname() it once it's done.
>
> Or just do __getname() in the caller and pass it to the function -
> in that case freeing (in all cases) would be up to the caller.

Thanks for pointing this out. Someone should look at this closely and
clean it up.

--
Best regards,
Pavel Shilovsky