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Sat, 22 Dec 2018 04:40:01 -0800

On 2018-12-21 12:58, Tim R¨¹hsen wrote:
> On 12/21/18 12:09 PM, Aurelien Jarno wrote:
> > On 2018-12-21 11:51, Tim R¨¹hsen wrote:
> >> On 12/19/18 12:55 AM, Aurelien Jarno wrote:
> >>> On 2018-12-18 22:11, Aurelien Jarno wrote:
> >>>> On 2018-12-18 21:34, Aurelien Jarno wrote:
> >>>>> Hi,
> >>>>>
> >>>>> On 2018-12-18 15:15, Tim Ruehsen wrote:
> >>>>>> Package: libc6-armhf-cross
> >>>>>> Version: 2.28-2cross2
> >>>>>> Severity: normal
> >>>>>>
> >>>>>> Dear Maintainer,
> >>>>>>
> >>>>>> currently strerror(-3) sets errno unexpectedly to ENOMEM (12).
> >>>>>>
> >>>>>> The expected errno value would be either EINVAL or not touching errno
> >>>>>> at all.
> >>>>>>
> >>>>>> This behavior is relatively new and causes some CI cross builds to 
> >>>>>> fail.
> >>>>>> The failing test is a gnulib test (test-strerror.c).
> >>>>>>
> >>>>>
> >>>>> I can reproduce the issue with libc6-armhf-cross 2.28-2cross2 and
> >>>>> qemu-arm-static 1:3.1+dfsg-1, but not with the same binary on real
> >>>>> hardware nor on qemu-user-static 1:2.12+dfsg-3+b1. I would therefore
> >>>>> think it's a qemu bug.
> >>>>
> >>>> Hmm, I am wrong, I can actually reproduce it with qemu-user-static
> >>>> version 1:2.12+dfsg-3+b1. But I can't reproduce it on real hardware.
> >>>
> >>> It seems to have been caused by this upstream patch:
> >>>
> >>> | commit 1294b1892e19d70e9e4dca0a2f3e39497f262a42
> >>> | Author: Wilco Dijkstra <wdijk...@arm.com>
> >>> | Date:   Thu Mar 15 17:57:03 2018 +0000
> >>> | 
> >>> |     Add support for sqrt asm redirects
> >>> |     
> >>> |     This patch series cleans up the many uses of  __ieee754_sqrt(f/l) 
> >>> in GLIBC.
> >>> |     The goal is to enable GCC to do the inlining, and if this fails 
> >>> call the
> >>> |     __ieee754_sqrt function.  This is done by internally declaring sqrt 
> >>> with asm
> >>> |     redirects.  The compat symbols and sqrt wrappers need to disable 
> >>> the redirect.
> >>> |     The redirect is also disabled if there are already redirects 
> >>> defined when
> >>> |     using -ffinite-math-only.
> >>> |     
> >>> |     All math functions (but not math tests, non-library code and 
> >>> libnldbl) are
> >>> |     built with -fno-math-errno which means GCC will typically inline 
> >>> sqrt as a
> >>> |     single instruction.  This means targets are no longer forced to add 
> >>> a special
> >>> |     inline for sqrt.
> >>> |     
> >>> |             * include/math.h (sqrt): Declare with asm redirect.
> >>> |             (sqrtf): Likewise.
> >>> |             (sqrtl): Likewise.
> >>> |             (sqrtf128): Likewise.
> >>> |             * Makeconfig: Add -fno-math-errno for libc/libm, but build 
> >>> testsuite,
> >>> |             nonlib and libnldbl with -fmath-errno.
> >>> |             * math/w_sqrt_compat.c: Define NO_MATH_REDIRECT.
> >>> |             * math/w_sqrt_template.c: Likewise.
> >>> |             * math/w_sqrtf_compat.c: Likewise.
> >>> |             * math/w_sqrtl_compat.c: Likewise.
> >>> |             * sysdeps/i386/fpu/w_sqrt.c: Likewise.
> >>> |             * sysdeps/i386/fpu/w_sqrt_compat.c: Likewise.
> >>> |             * sysdeps/generic/math-type-macros-float128.h: Remove 
> >>> math.h and
> >>> |             complex.h.
> >>>
> >>> And more precisely by building libc with -fno-math-errno.
> >>
> >> Thanks for looking into it.
> >>
> >> How is the proceeding ? Is there enough info to fix (or report upstream)
> >> ? If not, what has to be done ?
> > 
> > No it's not enough to fix it or report it upstream. We still have to
> > understand the exact issue. For me it's not yet clear if the bug is in
> > QEMU or in glibc. The fact that it works fine on real hardware would
> > go towards a QEMU bug, but there is no proof yet.
> 
> Looking at glibc's string/strerror.c, it calls __strerror_r() before
> saving errno.
> 
> In __strerror_r(), gettext() is being called via a the #define _().
> 
> gettext() saves/restores errno only if successful, else it doesn't.
> __strerror_r() doesn't check or save errno at all.
> 
> So whenever gettext() sets errno, this value stays when strerror()
> returns. The gettext() code path is only travelled when errnum is < 0.
> 
> You can of course argue, if gettext() or strerror() must be fixed. But
> that is clearly an upstream issue.
> 
> And if there is an underlying issue with memory allocation is a
> different issue. But is doesn't affect the strerror() function in the
> gnulib test as it seems.

This is not what happens, errno is not set to ENOMEM in strerror_() but
in strerror(). The problem is that the malloc implementation when run
under QEMU sets errno to ENOMEM, despite successfully allocating the
memory. errno is supposed to be saved around the malloc call:

  saved_errno = errno;
  if (buf == NULL) 
    buf = malloc (1024);
  __set_errno (saved_errno); 

That said, when compiled with -fno-math-errno, GCC optimizes out
saving and restoring errno around the malloc call. I am not sure if this
is a GCC bug or a bug in the GCC documentation.

Note that the fact that malloc() successfully allocates memory but still
sets errno to ENOMEM might also be due to the use of -fno-math-errno.

Regards,
Aurelien

-- 
Aurelien Jarno                          GPG: 4096R/1DDD8C9B
aurel...@aurel32.net                 free online bettinghttp://www.aurel32.net

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