From: Pavel Shilovsky <piastryyy@gmail.com>
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
Steve French <stfrench@microsoft.com>,
Ronnie Sahlberg <lsahlber@redhat.com>,
linux-cifs <linux-cifs@vger.kernel.org>,
Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: build_path_from_dentry_optional_prefix() may schedule from invalid context
Date: Mon, 30 Sep 2019 10:32:16 -0700 [thread overview]
Message-ID: <CAKywueSC=MoBB6t2OeUiyc6+GST2Jgg8FTO-kkXif-pn+1k-cw@mail.gmail.com> (raw)
In-Reply-To: <20190921223847.GB29065@ZenIV.linux.org.uk>
сб, 21 сент. 2019 г. в 15:38, Al Viro <viro@zeniv.linux.org.uk>:
>
> 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
next prev parent reply other threads:[~2019-09-30 21:38 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-29 5:02 build_path_from_dentry_optional_prefix() may schedule from invalid context Sergey Senozhatsky
2019-09-20 0:11 ` Pavel Shilovsky
2019-09-21 22:38 ` Al Viro
2019-09-30 17:32 ` Pavel Shilovsky [this message]
2019-12-09 0:34 ` Al Viro
2019-12-10 19:14 ` Pavel Shilovsky
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