[cfe-dev] RFC: default to -Werror=format-security

[Bob Wilson via cfe-dev]

This sounds reasonable to me. Thanks for all the feedback.

I’m testing a patch to add a fix-it and I found some other things to clean up related to this.

> On Feb 17, 2016, at 3:25 PM, Nico Weber via cfe-dev <cfe-dev at lists.llvm.org> wrote:
> 
> Let me attempt a summary here:
> 
> * For the specific warning, it sounds like making it more useful (give it a fixit, improve error text -- see the bug that was just linked to, etc) will likely make people pay more attention to it
> * It sounds like a "warning level" concept is considered useful by some since there's too much stuff on by default (?) and the jump from "default warnings" to "-Wall" is too big. From what I gathered, people want something more granular than default, -wall, -wextra, but something less granular than turning individual groups on and off.
> * Some people think it might make sense to turn on warnings-as-errors for lower warning levels.
> * But that's contested, and it's also not clear if -Wformat-security would be in a low warning level category.
> 
> So I think the consensus to the question in the original post is what Reid said ("My feeling is that we shouldn't do this in upstream clang. [...] think it's totally reasonable for vendors to want to make this default to an error, though").
> 
> On Wed, Feb 17, 2016 at 3:04 PM, Alexander Riccio via cfe-dev <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>> wrote:
> I think the recent bug I opened up is pertinent, as it's not clearly (for some people, like me) bad code at fault:
> 
> https://llvm.org/bugs/show_bug.cgi?id=26643 <https://llvm.org/bugs/show_bug.cgi?id=26643>
> 
> sent from my (stupid) windows phone
> From: Craig, Ben via cfe-dev <mailto:cfe-dev at lists.llvm.org>
> Sent: ‎2/‎17/‎2016 4:10 PM
> To: Sean Silva <mailto:chisophugis at gmail.com>; Aaron Ballman <mailto:aaron at aaronballman.com>
> Cc: cfe-dev <mailto:cfe-dev at lists.llvm.org>
> Subject: Re: [cfe-dev] RFC: default to -Werror=format-security
> 
> On 2/17/2016 3:03 PM, Sean Silva via cfe-dev wrote:
>> On Wed, Feb 17, 2016 at 5:27 AM, Aaron Ballman via cfe-dev <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>> wrote:
>> On Wed, Feb 17, 2016 at 3:48 AM, David Chisnall
>> <David.Chisnall at cl.cam.ac.uk <mailto:David.Chisnall at cl.cam.ac.uk>> wrote:
>> > On 16 Feb 2016, at 21:56, Aaron Ballman via cfe-dev <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>> wrote:
>> >>
>> >> Sorry, but printf(fmt); is *always* a true positive in my book. Same
>> >> with failing to return from all code paths. (etc)
>> >
>> > You are wrong.  The most common reason for printf(fmt) to appear is that fmt is the result of doing a lookup of the locale-aware version of some constant string.  In this case, the contents of fmt is entirely under the control of whoever shipped the application, and will have been checked for format string vulnerabilities by the localisation tools (at least, assuming that the original that is being translated are free from vulnerabilities).  If you are not doing any caching in the application, then you can mark the translation function with the attribute that indicates that its input and output have the same format string compatibility.  If you are caching, then there is no easy way of silencing this warning.
>> >
>> > Making this an error will cause valid and correct code to fail to compile and will result in people simply disabling the warning, rather than checking it.
>> 
>> If the expected string does not have any format specifiers, then
>> printf("%s", fmt) is definitely the correct way to write that because
>> the assumption "entirely under the control of whoever shipped the
>> application" is a poor one. If it does have format specifiers, I agree
>> that we should not err, but I don't believe that was on the table.
>> 
>> I think David is talking about a situation where it is e.g.
>> 
>> printf(translate("Please enter a number from %d-%d\n"), lo, hi);
>> 
> 
> Note from the original post:
>     "This warning complains about a printf-like format string that is not a literal string and is used without any arguments."
> That means that 'printf(translate("Please press OK to continue"));' would trigger this warning (rightfully).  But the example you gave would not trigger the warning, as the invocation has extra 'lo' and 'hi' arguments.
> 
>> -- Sean Silva
>>  
>> 
>> ~Aaron
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