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Date: Fri, 1 Jun 2018 00:36:07 -0700
From: Andrei Vagin <avagin@...il.com>
To: Rich Felker <dalias@...c.org>
Cc: musl@...ts.openwall.com
Subject: Re: Re: [PATCH] scanf: handle the L modifier for integers

On Thu, May 31, 2018 at 07:44:36PM -0400, Rich Felker wrote:
> On Thu, May 31, 2018 at 10:44:42PM +0200, Natanael Copa wrote:
> > On Thu, 31 May 2018 12:00:22 -0700
> > Andrei Vagin <avagin@...tuozzo.com> wrote:
> > 
> > > >>Without this patch, ret will be 1 and mask will be 0. It is obviously
> > > >>incorrect. According to the man page, L should work like ll:
> > > >>
> > > >>L Indicates that the conversion will be either e, f, or g and the
> > > >>   next pointer is a pointer to long double or the conversion will
> > > >>   be d, i, o, u, or x and the next pointer is a pointer to long
> > > >>   long.  
> > > >
> > > >  This is a GNU extension. POSIX states that L is only valid before
> > > >a floating-point conversion specifier:
> > > >
> > > >L
> > > >     Specifies that a following a, A, e, E, f, F, g, or G conversion 
> > > >specifier
> > > >     applies to an argument with type pointer to long double.
> > > >
> > > >  from 
> > > >http://pubs.opengroup.org/onlinepubs/9699919799/functions/scanf.html
> > > >
> > > >  So, it is valid for musl not to accept %Lx.
> > > >  Now, the argument that it's a good idea to align musl's behaviour to
> > > >glibc's whenever possible is a sensible one. But it's a decision for
> > > >the musl authors to make, and the pros and cons need to be carefully
> > > >balanced; musl's current behaviour is not _incorrect_.  
> > > 
> > > It is incorrect, because scanf() has to return 0, or it has to handle the
> > > L modifier. Currently it doesn't handle L and return 1, so the
> > > application can't detect this issue.
> > 
> > That sounds like a bug in musl libc.
> >  
> > > I would prefer a case when musl works like glibc, if there are not any
> > > reason to not to do that. For example,  now Alpine Linux is very popular
> > > and there are a lot of packages. In many cases, a maintainer, who adds a
> > > new package, fixes compile-time errors and doesn't run any tests.
> > > A target application can work differently with musl comparing with glibc
> > > due to this sort of issues.
> > 
> > FreeBSD man page says:
> > 
> >      L	      Indicates	that the conversion will be one	of a, e, f, or g and
> > 	      the next pointer is a pointer to long double.
> > 
> > NetBSD man page says:
> > 
> >      L       Indicates that the conversion will be efg and the next pointer is
> >              a pointer to long double.
> > 
> > OpenBSD man page says:
> >      
> > L
> >     Indicates that the conversion will be one of efg and the next pointer is a pointer to long double.
> > 
> > So the application will break on most (every) system that is not GNU
> > libc. It would be better to fix the application in this case:
> > 
> > 
> >    char str[] = "sigmask: 0x200";
> >    long long mask = 0;
> >    int ret;
> > 
> > #if defined(__GLIBC__)
> >    ret = sscanf(str, "sigmask: %Lx", &mask));
> > #else
> >    ret = sscanf(str, "sigmask: %llx", &mask));
> > #endif
> >    printf("%d %llx\n", ret, mask);
> > 
> > 
> > 
> > Or just use %llx which is POSIX and should work everywhere.
> 
> Indeed, there is no reason to use %Lx anywhere. It's simply wrong.
> 
> > That said, those things are tricky to detect at compile time as you
> > mentioned and they are tricky to detect with configure scripts that
> > works with cross compilation.
> 
> If gcc does not catch this with -Wformat, it's a gcc bug that we
> should report and try to get fixed. It's possible that they're making
> an exception for the invalidity of L with integer formats since some
> libcs support that, but I don't see any good reason for this; gcc
> should still be warning about the incorrect and nonportable usage. I
> can't imagine they'd be opposed to a patch to fix it.

I found that gcc catches this with -pedantic -Wformat:

/musl # gcc -Wall -pedantic test.c
test.c: In function 'main':
test.c:9:28: warning: ISO C does not support the '%Lx' gnu_scanf format [-Wformat=]
  ret = sscanf(str, "%llx %Lx", &a, &b);

> 
> > Also many developers seems to think that
> > Linux == glibc so they only read the GNU manuals, so yeah, implement
> > glibc behavior here seems like a good idea, unless someone else has a
> > brilliant idea how to catch this at compile time.
> 
> Aside from fixing gcc at compile time, this has come up before (with
> regard to printf, not scanf), and my leaning then and now was to
> detect the UB at runtime by crashing rather than reporting an error as
> we do now, since (1) it's UB, so an application can't reasonably
> expect an error, and (2) applications seem to be ignoring errors
> anyway.
> 
> We should also get the man page fixed. The printf man page is clear
> that L with integer specifiers is a nonstandard extension and should
> not be used (they're not documented under L, only as a note at the
> end) but it seems whoever fixed this overlooked changing scanf at the
> same time.
> 
> Rich

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