Subject: Re: [PATCH v2 3/3] lib/string_helpers.c: Change semantics of string_escape_mem

From: Andy Shevchenko <[email-masked]>
Date: Fri, 30 Jan 2015 12:27:35 +0200

On Thu, 2015-01-29 at 15:29 +0100, Rasmus Villemoes wrote:
> On Thu, Jan 29 2015, Andy Shevchenko <[email-masked]> wrote:
> 
> >>   *
> >>   * Return:
> >> - * The amount of the characters processed to the destination buffer, or
> >> - * %-ENOMEM if the size of buffer is not enough to put an escaped character is
> >> - * returned.
> >> - *
> >> - * Even in the case of error @dst pointer will be updated to point to the byte
> >> - * after the last processed character.
> >> + * The total size of the escaped output that would be generated for
> >> + * the given input and flags. To check whether the output was
> >> + * truncated, compare the return value to osz. There is room left in
> >> + * dst for a '\0' terminator if and only if ret < osz.
> >>   */
> >> -int string_escape_mem(const char *src, size_t isz, char **dst, size_t osz,
> >> -		      unsigned int flags, const char *esc)
> >> +size_t string_escape_mem(const char *src, size_t isz, char *dst, size_t osz,
> >> +			 unsigned int flags, const char *esc)
> >
> > I prefer to leave the prototype the same. int for return is okay. dst
> > should be updated accordingly.
> 
> Please explain exactly what you think the return value should be, and
> what *dst should be set to.

Return value like you proposed, *dst is incremented by it.

> 
> >>  {
> >> -	char *p = *dst;
> >> +	char *p = dst;
> >>  	char *end = p + osz;
> >>  	bool is_dict = esc && *esc;
> >> -	int ret;
> >>  
> >>  	while (isz--) {
> >>  		unsigned char c = *src++;
> >> @@ -466,13 +463,7 @@ int string_escape_mem(const char *src, size_t isz, char **dst, size_t osz,
> >>  
> >>  		escape_passthrough(c, &p, end);
> >>  	}
> >> -	if (p > end) {
> >> -		*dst = end;
> >> -		return -ENOMEM;
> >> -	}
> >>  
> >> -	ret = p - *dst;
> >> -	*dst = p;
> >> -	return ret;
> >> +	return p - dst;
> >>  }
> >>  EXPORT_SYMBOL(string_escape_mem);
> >> diff --git a/lib/test-string_helpers.c b/lib/test-string_helpers.c
> >> index ab0d30e1e18f..5f95114a2f86 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;
> >> +	size_t p = 0, q_test = 0;
> >> +	size_t 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 %zu, got %zu\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);
> >
> > Could it be a part of nomem test still?
> 
> What nomem test? string_escape_mem with snprintf-like semantics cannot
> return an error; that has to be checked by the caller. 

Make this code a separate test, which actually still nomem, since you
have not enough memory in the destination buffer. What the problem to
check for proper return value and the last couple of characters written
to the destination buffer?

> 
> >> +
> >>  out:
> >>  	kfree(in);
> >>  	kfree(out_real);
> >>  	kfree(out_test);
> >>  }
> >>  
> >> -static __init void test_string_escape_nomem(void)
> >> -{
> >> -	char *in = "\eb \\C\007\"\x90\r]";
> >> -	char out[64], *buf = out;
> >> -	int rc = -ENOMEM, ret;
> >> -
> >> -	ret = string_escape_str_any_np(in, &buf, strlen(in), NULL);
> >> -	if (ret == rc)
> >> -		return;
> >> -
> >> -	pr_err("Test 'escape nomem' failed: got %d instead of %d\n", ret, rc);
> >> -}
> >> -
> >>  static int __init test_string_helpers_init(void)
> >>  {
> >>  	unsigned int i;
> >> @@ -342,8 +339,6 @@ static int __init test_string_helpers_init(void)
> >>  	for (i = 0; i < (ESCAPE_ANY_NP | ESCAPE_HEX) + 1; i++)
> >>  		test_string_escape("escape 1", escape1, i, TEST_STRING_2_DICT_1);
> >>  
> >> -	test_string_escape_nomem();
> >> -
> >>  	return -EINVAL;
> >>  }
> >>  module_init(test_string_helpers_init);
> >> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> >> index 3568e3906777..d02c394b5b58 100644
> >> --- a/lib/vsprintf.c
> >> +++ b/lib/vsprintf.c
> >> @@ -1160,7 +1160,7 @@ char *escaped_string(char *buf, char *end, u8 *addr, struct printf_spec spec,
> >>  	len = spec.field_width < 0 ? 1 : spec.field_width;
> >>  
> >>  	/* Ignore the error. We print as many characters as we can */
> >> -	string_escape_mem(addr, len, &buf, end - buf, flags, NULL);
> >> +	buf += string_escape_mem(addr, len, buf, buf < end ? end - buf : 0, flags, NULL);
> >
> > So, the problem is when we have end < buf, right?
> > How about to move this check out of the call parameters?
> >
> > [Keep in might the original prototype]
> >
> > if (buf < end)
> >  string_escape_mem(addr, len, &buf, end - buf, flags, NULL);
> > else
> >  string_escape_mem(addr, len, &buf, 0, flags, NULL);
> 
> In that case, I just did the same as is done for %pV, and prefer to keep
> it that way.

I've checked the other case, we may keep same style.

> >> diff --git a/net/sunrpc/cache.c b/net/sunrpc/cache.c
> >> index 33fb105d4352..22c4418057f4 100644
> >> --- a/net/sunrpc/cache.c
> >> +++ b/net/sunrpc/cache.c
> >> @@ -1068,12 +1068,14 @@ void qword_add(char **bpp, int *lp, char *str)
> >>  {
> >>  	char *bp = *bpp;
> >>  	int len = *lp;
> >> -	int ret;
> >> +	int ret, written;
> >>  
> >>  	if (len < 0) return;
> >>  
> >> -	ret = string_escape_str(str, &bp, len, ESCAPE_OCTAL, "\\ \n\t");
> >> -	if (ret < 0 || ret == len)
> >> +	ret = string_escape_str(str, bp, len, ESCAPE_OCTAL, "\\ \n\t");
> >> +	written = min(ret, len);
> >> +	bp += written;
> >> +	if (ret >= len)
> >>  		len = -1;
> >>  	else {
> >>  		len -= ret;
> >
> > For this part the comment from J. Bruce is needed.
> >
> > There is one more user, i.e. fs/proc/array.c::task_name().
> >
> > In all of them we have to amend a prepend commentary. Like changing
> > 'Ignore the error' to 'Ignore the overflow'.
> 
> I hadn't looked for users in -next. I'll leave it to you to amend that
> patch before it hits mainline.

When your series will be ready (and actually I recommend to push first
patch apart from the rest since it's not related) I may do the update
for fs/proc/array.c.

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