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Re: Enabling some -Werror=format* by default?

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On Mon, Jun 10, 2024 at 07:48:59AM +0200, Helmut Grohne wrote:
> > We recently increased the time_t size on certain architectures and some
> > packages started failing to build because they were using a format
> > specifier too narrow for the wider time_t, e.g. #1067829.
> > But the only reason those package FTBFS is they enable either -Werror or
> > some more specific non-default switch like -Werror=format=2, so I suspect
> > much more packages contain similar code but gained only a warning. Isn't
> > this a bad thing? Should we enable at least some combination of -Wformat*
> > switches by default? Should we at least add a new flag to dpkg-buildflags
> > and do some test rebuilds with it enabled?
> 
> It wasn't quite clear to me what -Werror=format=2 actually means.
> According to the gcc documentation[1], -Wformat=2 currently means:
> 
>     -Wformat -Wformat-nonliteral -Wformat-security -Wformat-y2k.
> 
> Of these, we already enable -Werror=format-security, but not the other
> ones. It is not clear to me, which of these actually catches the the
> type mismatches. Would you do more research here?

#include <stdio.h>

void foo()
{
    printf("%d", 0L);
}

This produces a warning with just -Wformat ("format ‘%d’ expects argument
of type ‘int’, but argument 2 has type ‘long int’ [-Wformat=]"). It
doesn't look like any more narrow options for this exist (-Wno-* ones
listed in the -Wformat description don't silence this one, and they don't
look like they should anyway).

But I don't know if even -Werror=format is too much to enable by default
globally (I assume it's a good thing to enable it, though).

> It also is unclear how this impacts the archive and yes, I'd recommend a
> rebuild. Note though that we likely need this rebuild both on a 64bit
> architecture and a 32bit architecture that is not i386 (due to how t64
> works). A partial archive rebuild may work to gauge the size of the
> problem.
> 
> I note that this kind of change affects cross builds, so performing
> cross builds for armhf on amd64 will likely show many of these failures
> (in addition to all the regular cross build failures).
> 
> I recommend doing more research before moving forward with this. In
> particular a MBF about introduced problems would be prudent before doing
> the switch and even if we don't switch, such a MBF bears value on its
> own.
Do you think it makes sense to add this a flag that enables -Werror=format
to dpkg-buildflags(1), before, or after a test rebuild, before, or after
the MBF if we do one?

-- 
WBR, wRAR

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Thread

Enabling some -Werror=format* by default? Andrey Rakhmatullin <wrar@debian.org> - 2024-06-06 21:20 +0200
  Re: Enabling some -Werror=format* by default? Helmut Grohne <helmut@subdivi.de> - 2024-06-10 12:40 +0200
    Re: Enabling some -Werror=format* by default? Andrey Rakhmatullin <wrar@debian.org> - 2024-06-10 13:10 +0200
      Re: Enabling some -Werror=format* by default? Guillem Jover <guillem@debian.org> - 2024-06-10 16:30 +0200
        Re: Enabling some -Werror=format* by default? Andrey Rakhmatullin <wrar@debian.org> - 2024-06-10 17:10 +0200
          Re: Enabling some -Werror=format* by default? Guillem Jover <guillem@debian.org> - 2024-06-10 18:30 +0200
        Re: Enabling some -Werror=format* by default? Andrey Rakhmatullin <wrar@debian.org> - 2024-06-10 17:10 +0200
        Re: Enabling some -Werror=format* by default? Simon McVittie <smcv@debian.org> - 2024-06-10 17:10 +0200
      Re: Enabling some -Werror=format* by default? Helmut Grohne <helmut@subdivi.de> - 2024-06-10 18:30 +0200
        Re: Enabling some -Werror=format* by default? Fay Stegerman <flx@obfusk.net> - 2024-06-11 03:00 +0200
        Re: Enabling some -Werror=format* by default? Guillem Jover <guillem@debian.org> - 2024-06-11 12:10 +0200

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