Subject: Re: [PATCH v3 3/3] lib/string_helpers.c: Change semantics of string_escape_mem

From: Andy Shevchenko <andriy.shevchenko-VuQAYsv1563Yd54FQh9/[email-masked]>
Date: Tue, 10 Feb 2015 16:22:30 +0200

On Tue, 2015-02-10 at 14:02 +0100, Rasmus Villemoes wrote:
> On Tue, Feb 10 2015, Andy Shevchenko <andriy.shevchenko-VuQAYsv1563Yd54FQh9/[email-masked]> wrote:
> 
> > On Tue, 2015-02-10 at 00:44 +0100, Rasmus Villemoes wrote:
> >> The current semantics of string_escape_mem are inadequate for one of
> >> its current users, vsnprintf(). If that is to honour its contract, it
> >> must know how much space would be needed for the entire escaped
> >> buffer, and string_escape_mem provides no way of obtaining that (short
> >> of allocating a large enough buffer (~4 times input string) to let it
> >> play with, and that's definitely a big no-no inside vsnprintf).
> >> 
> >> So change the semantics for string_escape_mem to be more
> >> snprintf-like: Return the size of the output that would be generated
> >> if the destination buffer was big enough, but of course still only
> >> write to the part of dst it is allowed to, and don't do
> >> '\0'-termination. It is then up to the caller to detect whether output
> >> was truncated and to append a '\0' if desired. Also, we must output
> >> partial escape sequences, otherwise a call such as snprintf(buf, 3,
> >> "%1pE", "\123") would cause printf to write a \0 to buf[2] but leaving
> >> buf[0] and buf[1] with whatever they previously contained.
> >> 
> >> This also fixes a bug in the escaped_string() helper function, which
> >> used to unconditionally pass a length of "end-buf" to
> >> string_escape_mem(); since the latter doesn't check osz for being
> >> insanely large, it would happily write to dst. For example,
> >> kasprintf(GFP_KERNEL, "something and then %pE", ...); is an easy way
> >> to trigger an oops.
> >> 
> >> In test-string_helpers.c, I removed the now meaningless -ENOMEM test,
> >> and replaced it with testing for getting the expected return value
> >> even if the buffer is too small. Also ensure that nothing is written
> >> when osz == 0.
> >> 
> >> In net/sunrpc/cache.c, I think qword_add still has the same
> >> semantics. Someone should definitely double-check this.
> >
> > Thanks for an update. My comments below.
> > After addressing 'em, wrt changes to patch 2/3, take my 
> > Acked-by: Andy Shevchenko <andriy.shevchenko-VuQAYsv1563Yd54FQh9/[email-masked]>
> >
> > for all parts except net/sunrpc/cache.c.
> >
> >> 
> >> Signed-off-by: Rasmus Villemoes <linux-qQsb+v5E8BnlAoU/[email-masked]>
> >> ---
> >> index ab0d30e1e18f..5f759c3c2f60 100644
> >> --- a/lib/test-string_helpers.c
> >> +++ b/lib/test-string_helpers.c
> >> @@ -264,12 +264,12 @@ static __init void test_string_escape(const char *name,
> >>  				      const struct test_string_2 *s2,
> >>  				      unsigned int flags, const char *esc)
> >>  {
> >> -	int q_real = 512;
> >> -	char *out_test = kmalloc(q_real, GFP_KERNEL);
> >> -	char *out_real = kmalloc(q_real, GFP_KERNEL);
> >> +	size_t out_size = 512;
> >> +	char *out_test = kmalloc(out_size, GFP_KERNEL);
> >> +	char *out_real = kmalloc(out_size, GFP_KERNEL);
> >>  	char *in = kmalloc(256, GFP_KERNEL);
> >> -	char *buf = out_real;
> >>  	int p = 0, q_test = 0;
> >> +	int q_real;
> >>  
> >>  	if (!out_test || !out_real || !in)
> >>  		goto out;
> >> @@ -301,29 +301,26 @@ static __init void test_string_escape(const char *name,
> >>  		q_test += len;
> >>  	}
> >>  
> >> -	q_real = string_escape_mem(in, p, &buf, q_real, flags, esc);
> >> +	q_real = string_escape_mem(in, p, out_real, out_size, flags, esc);
> >>  
> >>  	test_string_check_buf(name, flags, in, p, out_real, q_real, out_test,
> >>  			      q_test);
> >> +
> >> +	memset(out_real, 'Z', out_size);
> >> +	q_real = string_escape_mem(in, p, out_real, 0, flags, esc);
> >> +	if (q_real != q_test)
> >> +		pr_warn("Test '%s' failed: flags = %u, osz = 0, expected %d, got %d\n",
> >> +			name, flags, q_test, q_real);
> >> +	if (memchr_inv(out_real, 'Z', out_size))
> >> +		pr_warn("Test '%s' failed: osz = 0 but string_escape_mem wrote to the buffer\n",
> >> +			name);
> >> +
> >
> > So, why couldn't we split this to separate test case? It seems I already
> > pointed this out.
> >
> 
> This actually provides better coverage

I do not see much advantage of doing so. You may create a loop with
random number for in-size and check. So, I prefer to see separate case
for that.

>  since we do this for all the
> "positive" test cases, instead of just the single ad hoc case done previously. Of
> course the added lines could be factored into a separate helper, but
> there's quite a lot of state to pass, so I thought this would actually
> be simpler - note how the two string_escape_mem calls are easily seen to
> be identical except for the outsize argument.
> 
> It may already be too late for the merge window, but I didn't want to
> spend too much time on these mostly cosmetic details (that also goes for
> the 3- versus 2-line issue).

Yes, too late, thus it's enough time to address my comments :-)

On the other hand I actually don't know if it's a good idea to push this
series to stable. I guess you may just put Fixes: tags in the patches
1/3, 3/3 w/o Cc'ing to stable since we have no issues with current
users.

-- 
Andy Shevchenko <[email-masked]>
Intel Finland Oy

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