Re: [RFC][PATCH] string.h: Add strncmp_prefix() helper macro

[Steven Rostedt]

Seems that I sent this as my daughter.

I'm not home now but will resend again as myself when I am.

-- Steve

On December 19, 2018 4:54:57 PM EST, Joe Perches <joe@xxxxxxxxxxx> wrote:
>On Wed, 2018-12-19 at 16:32 -0500, Bryana Rostedt wrote:
>> A discussion came up in the trace triggers thread about converting a
>> bunch of:
>> 
>>  strncmp(str, "const", sizeof("const") - 1)
>> 
>> use cases into a helper macro. It started with:
>> 
>> #define strncmp_const(str, const) \
>> 	strncmp(str, const, sizeof(const) - 1)
>> 
>> But then Joe Perches mentioned that if a const is not used, the
>> sizeof() will be the size of a pointer, which can be bad. And that
>> gcc will optimize strlen("const") into "sizeof("const") - 1".
>> 
>> Thinking about this more, a quick grep in the kernel tree found
>several
>> (thousands!) of cases that use this construct. A quick grep also
>> revealed that there's probably several bugs in that use case. Some
>are
>> that people forgot the "- 1" (which I found) and others could be that
>> the constant for the sizeof is different than the constant (although,
>I
>> haven't found any of those, but I also didn't look hard).
>> 
>> I figured the best thing to do is to create a helper macro and place
>it
>> into include/linux/string.h. And go around and fix all the open coded
>> versions of it later.
>> 
>> I plan on only applying this patch and updating the tracing hooks for
>> this merge window. And perhaps use it to fix some of the bugs that
>were
>> found.
>> 
>> I was going to just use:
>> 
>> #define strncmp_prefix(str, prefix) \
>> 	strncmp(str, prefix, strlen(prefix))
>> 
>> but then realized that "prefix" is used twice, and will break if
>> someone does something like:
>> 
>> 	strncmp_prefix(str, ptr++);
>> 
>> So instead I check with __builtin_constant_p() to see if the second
>> parameter is truly a constant, which I use sizeof() anyway (why
>bother
>> gcc to optimize it, if we already know it's a constant), and copy the
>> parameter into a local variable and use that local variable for the
>non
>> constant part (with strlen).
>> 
>> Link:
>http://lkml.kernel.org/r/e3e754f2bd18e56eaa8baf79bee619316ebf4cfc.1545161087.git.tom.zanussi@xxxxxxxxxxxxxxx
>> 
>> Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
>> ---
>> diff --git a/include/linux/string.h b/include/linux/string.h
>> index 27d0482e5e05..3dc743e3a0ba 100644
>> --- a/include/linux/string.h
>> +++ b/include/linux/string.h
>> @@ -14,6 +14,27 @@ extern void *memdup_user(const void __user *,
>size_t);
>>  extern void *vmemdup_user(const void __user *, size_t);
>>  extern void *memdup_user_nul(const void __user *, size_t);
>>  
>> +/*
>> + * A common way to test a prefix of a string is to do:
>> + *  strncmp(str, prefix, sizeof(prefix) - 1)
>> + *
>> + * But this can lead to bugs due to typos, or if prefix is a pointer
>> + * and not a constant. Instead use strncmp_prefix().
>> + */
>> +#define strncmp_prefix(str, prefix)					\e
>> +	({								\
>> +		int ____strcmp_prefix_ret____;				\
>> +		char *____strcmp_prefix____ = prefix;			\
>
>This creates a warning and discards any const from prefix when used
>like
>
>	static const char foo[] = "bar";
>
>	strncmp_prefix(str, foo);
>
>> +		if (__builtin_constant_p(&prefix))			\
>> +			____strcmp_prefix_ret____ =			\
>> +				strncmp(str, prefix, sizeof(prefix) - 1); \
>> +		else							\
>> +			____strcmp_prefix_ret____ =			\
>> +				strncmp(str, ____strcmp_prefix____,	\
>> +					strlen(____strcmp_prefix____));	\
>> +		____strcmp_prefix_ret____;				\
>> +	})
>> +
>
>Perhaps
>
>#define strncmp_prefix(str, prefix)					\
>({									\
>	typeof(prefix) p = (prefix);					\
>	strncmp(str, p, strlen(p));					\
>})

-- 
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