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From: "jakub at redhat dot com" <sourceware-bugzilla@sourceware.org>
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Subject: [Bug libc/15813] Multiple issues in __gen_tempname
Date: Wed, 09 Sep 2020 09:37:51 +0000
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D15813

Jakub Jelinek <jakub at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |REOPENED
         Resolution|FIXED                       |---
                 CC|                            |jakub at redhat dot com

--- Comment #6 from Jakub Jelinek <jakub at redhat dot com> ---
(In reply to cvs-commit@gcc.gnu.org from comment #4)
> The master branch has been updated by Adhemerval Zanella
> <azanella@sourceware.org>:
>=20
> https://sourceware.org/git/gitweb.cgi?p=3Dglibc.git;
> h=3De1df30fbc2e2167a982c0e77a7ebee28f4dd0800
>=20
> commit e1df30fbc2e2167a982c0e77a7ebee28f4dd0800
> Author: Adhemerval Zanella <adhemerval.zanella@linaro.org>
> Date:   Thu Jul 25 11:22:17 2019 -0300
>=20
>     Get new entropy on each attempt __gen_tempname (BZ #15813)
>=20=20=20=20=20
>     This is missing bit for fully fix BZ#15813 (the other two were fixed
>     by 359653aaacad463).
>=20=20=20=20=20
>     Checked on x86_64-linux-gnu.
>=20=20=20=20=20
>     	[BZ #15813]
>     	sysdeps/posix/tempname.c (__gen_tempname): get entrypy on each
>     	attempt.

This change is completely broken.
As random_bits is defined as:
static inline uint32_t
random_bits (void)
{
  struct __timespec64 tv;
  __clock_gettime64 (CLOCK_MONOTONIC, &tv);
  /* Shuffle the lower bits to minimize the clock bias.  */
  uint32_t ret =3D tv.tv_nsec ^ tv.tv_sec;
  ret ^=3D (ret << 24) | (ret >> 8);
  return ret;
}
on a fast machine instead of making sure that in the loop the value is even
more random it makes sure that each attempt uses the exact same value, whic=
h it
knows from the earlier attempt that it exists already.
So, either it needs to ensure the entropy source is not imprecise time base=
d,
or it needs e.g. add to the initial value in each iteration rather than use=
 the
same value.

--=20
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