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Message-ID: <20240610160408.GN10433@brightrain.aerifal.cx> Date: Mon, 10 Jun 2024 12:04:08 -0400 From: Rich Felker <dalias@...c.org> To: Ismael Luceno <ismael@...ev.co.uk> Cc: musl@...ts.openwall.com Subject: Re: [PATCH] ioctl: Fix implicit constant conversion overflow On Mon, Jun 03, 2024 at 03:57:18AM +0200, Ismael Luceno wrote: > On 02/Jun/2024 18:50, Rich Felker wrote: > > On Sun, Jun 02, 2024 at 05:01:10AM +0200, Ismael Luceno wrote: > > > On 31/May/2024 22:34, Rich Felker wrote: > > > <...> > > > > > +#define _IOW(a,b,c) _IOC(_IOC_WRITE,(a),(b),(int)sizeof(c)) > > > > > +#define _IOR(a,b,c) _IOC(_IOC_READ,(a),(b),(int)sizeof(c)) > > > > > +#define _IOWR(a,b,c) _IOC(_IOC_READ|_IOC_WRITE,(a),(b),(int)sizeof(c)) > > > > > > > > I don't see how this helps with the warning you're trying to suppress, > > > > > > GCC disagrees; the warnings go away because it's this element that > > > causes the whole expression to be promoted to unsigned long long, > > > so making it smaller (we can use unsigned int instead) avoids the > > > issue. > > > > In that case gcc is just being inconsistent. Both the conversion from > > unsigned int to int and size_t to int are non-value-preserving. It > > makes no sense that it warns for the latter but not for the former. > > > > "Make weird inconsistent warning messages go away" is not a motivation > > for a change. If the command macros could all be made to have type int > > (matcing the ioctl argument) without introducing new problems, that > > would be a well-motivated change. I suppose "make them have type > > unsigned int rather than unsigned long so that they're not > > gratuitously over-wide" might be well-motivated too, but I suspect it > > leaves in place warnings in some places. "Fix implicit constant > > conversion overflow" is not a well-motivated change since there is no > > overflow. > > GCC doesn't make much sense here but the warning appears with several > versions of GCC. > > An explicit cast at _IOC instead would make sense to me, but what could > break in your opinion? I'm not sure. It needs investigation. There might have been some concern with breakage from kernel headers that define ioctl numbers or something. I just remember this hasn't been as simple as it sounds from past times it came up.. Rich
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