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Date: Mon, 4 Sep 2017 23:56:41 +0200
From: Bartosz Brachaczek <b.brachaczek@...il.com>
To: musl@...ts.openwall.com
Subject: Re: [PATCH] handle whitespace before %% in scanf
On 9/4/2017 10:59 PM, Rich Felker wrote:
> On Mon, Jul 10, 2017 at 09:20:39PM -0400, Rich Felker wrote:
>> On Sun, Jul 09, 2017 at 11:00:18PM +0200, Bartosz Brachaczek wrote:
>>> this is mandated by C and POSIX standards and is in accordance with
>>> glibc behavior.
>>> ---
>>> src/stdio/vfscanf.c | 10 +++++++---
>>> src/stdio/vfwscanf.c | 8 ++++++--
>>> 2 files changed, 13 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/src/stdio/vfscanf.c b/src/stdio/vfscanf.c
>>> index d4d2454b..9e030fc4 100644
>>> --- a/src/stdio/vfscanf.c
>>> +++ b/src/stdio/vfscanf.c
>>> @@ -89,15 +89,19 @@ int vfscanf(FILE *restrict f, const char *restrict fmt, va_list ap)
>>> continue;
>>> }
>>> if (*p != '%' || p[1] == '%') {
>>> - p += *p=='%';
>>> shlim(f, 0);
>>> - c = shgetc(f);
>>> + if (*p == '%') {
>>> + p++;
>>> + while (isspace((c=shgetc(f))));
>>> + } else {
>>> + c = shgetc(f);
>>> + }
>>> if (c!=*p) {
>>> shunget(f);
>>> if (c<0) goto input_fail;
>>> goto match_fail;
>>> }
>>> - pos++;
>>> + pos += shcnt(f);
>>> continue;
>>> }
>>
>> Assuming your interpretation is correct, I have no objection to going
>> forward with the change, but I don't think this is the right way to do
>> it. The only reason %% was handled in the code that handles literal
>> characters is because I assumed it behaves like one, but if it
>> doesn't, it should just be handled as a format specifier that consumes
>> space where it can use the existing code that does that, rather than
>> complicting the code for literals and adding a duplicate of the
>> space-skipping code to it.
>
> I tried going forward with the idea I proposed, but it looks like it's
> actually more invasive: in addition to adding the final case to
> actually handle '%', it adds a new case where a conversion specifier
> does not consume a variadic input, and a new case where width is
> forced to 1 and modifier flags and explicit widths are rejected.
>
> As such I think your patch as originally submitted is probably the
> best approach. Sorry for the delay in reviewing and accepting it.
Oh, that's perfect, thanks. Sorry I didn't get to responding to your
request. I originally tried both approaches and chose the one that had
smaller impact on code size in vfscanf.o.
While at it, you might want to have a look at another trivial patch for
vfwscanf I submitted:
http://www.openwall.com/lists/musl/2017/07/09/5
If anything, it avoids confusion for people reading the code. I'm
attaching a version of this patch with enough context lines for inline
review.
View attachment "0001-slightly-simplify-a-condition-in-vfwscanf.patch" of type "text/plain" (1004 bytes)
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