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[perl #128254] Taint-mode assert fail in Perl_magic_clearisa withoutother symptoms

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Dan Collins
May 27, 2016 00:29
[perl #128254] Taint-mode assert fail in Perl_magic_clearisa withoutother symptoms
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# New Ticket Created by  Dan Collins 
# Please include the string:  [perl #128254]
# in the subject line of all future correspondence about this issue. 
# <URL: >

Greetings Porters,

I have compiled bleadperl with the afl-gcc compiler using:

./Configure -Dusedevel -Dprefix='/usr/local/perl-afl' -Dcc='ccache afl-gcc' -Uuselongdouble -Duse64bitall -Doptimize=-g -Uversiononly -Uman1dir -Uman3dir -Dusequadmath -des
AFL_HARDEN=1 make && make test

And then fuzzed the resulting binary using:

AFL_NO_VAR_CHECK=1 afl-fuzz -i in -o out bin/perl @@

After reducing testcases using `afl-tmin` and performing additional minimization by hand, I have located the following testcase that triggers an assert fail in debug buids of the perl interpreter. The testcase is the file below. On normal builds, this runs normally (albeit with an expected warning). On debug builds, this returns an assert fail.

dcollins@nightshade64:~/perl$ ./perl -Ilib -tW -e '{@{*a::ISA}=undef*a::ISA;@a::ISA=0}'
Use of uninitialized value in list assignment at -e line 1.
dcollins@nightshade64:~/perl$ cd ../perldebug/

dcollins@nightshade64:~/perldebug$ ./perl -Ilib -tW -e '{@{*a::ISA}=undef*a::ISA;@a::ISA=0}'
Use of uninitialized value in list assignment at -e line 1.
perl: mg.c:1726: Perl_magic_clearisa: Assertion `((((_gvstash)->sv_flags & (0x00004000|0x00008000)) == 0x00008000) && (((svtype)((_gvstash)->sv_flags & 0xff)) == SVt_PVGV || ((svtype)((_gvstash)->sv_flags & 0xff)) == SVt_PVLV))' failed.

Debugging tool output is below. A git bisect was performed and reported the following.

5e267fb87fe17363909b2ed49f916dccf9939c3b is the first bad commit
commit 5e267fb87fe17363909b2ed49f916dccf9939c3b
Author: David Mitchell <>
Date:   Wed Oct 21 13:10:44 2015 +0100

    Always copy return values when exiting scope

    v5.14.0-642-g3ed94dc fixed certain instances where returning from a sub
    incorrectly returned the actual value rather than a copy, e.g.

        sub f { delete $foo{bar} }

    This was because if the value(s) being returned had SvTEMP set, copying
    was skipped. That commit added an extra condition to the skip test,
    SvREFCNT(sv) == 1.

    However, this applies equally well to other scope exits, for example

        do { ...; delete $foo{bar} }

    So this commits adds the RC==1 test to S_leave_common() too, which handles
    all the non-sub scope exits. As well as adding a test to do.t, it adds an
    additional test to sub.t, since the original tests, although they
    *detected* a non-copied return, didn't actually demonstrate a case where
    it was actually harmful.

    Note that S_leave_common() also sometimes skips on PADTMPs as well as
    TEMPs, so this commit as a side-effect also makes it copy PADTMPs unless
    their RC ==1. But since their RC should in fact almost always be 1 anyway,
    in practice it makes no difference.

:100644 100644 82189bb5c188e77800176fe8cfcb80b15bcb2226 d1229af7609c41d1caa7944f8d90eb6a2b12859c M       pp_ctl.c
:040000 040000 95d71e14d5e47e4db9a5149492215aa45b9ad6c8 f69fbca5ed4bd5f0595f03be7deda11cca858d43 M       t
bisect run success


dcollins@nightshade64:~/perldebug$ gdb --args ./perl -Ilib -tW -e '{@{*a::ISA}=undef*a::ISA;@a::ISA=0}'
GNU gdb (GDB) 7.10
Copyright (C) 2015 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-unknown-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
Find the GDB manual and other documentation resources online at:
For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from ./perl...done.
(gdb) run
Starting program: /home/dcollins/perldebug/perl -Ilib -tW -e \{@\{\*a::ISA\}=undef\*a::ISA\;@a::ISA=0\}
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/".
Use of uninitialized value in list assignment at -e line 1.
perl: mg.c:1726: Perl_magic_clearisa: Assertion `((((_gvstash)->sv_flags & (0x00004000|0x00008000)) == 0x00008000) && (((svtype)((_gvstash)->sv_flags & 0xff)) == SVt_PVGV || ((svtype)((_gvstash)->sv_flags & 0xff)) == SVt_PVLV))' failed.

Program received signal SIGABRT, Aborted.
0x00007ffff6cf9478 in raise () from /lib/x86_64-linux-gnu/
(gdb) bt
#0  0x00007ffff6cf9478 in raise () from /lib/x86_64-linux-gnu/
#1  0x00007ffff6cfa8fa in abort () from /lib/x86_64-linux-gnu/
#2  0x00007ffff6cf23a7 in ?? () from /lib/x86_64-linux-gnu/
#3  0x00007ffff6cf2452 in __assert_fail () from /lib/x86_64-linux-gnu/
#4  0x000000000057548f in Perl_magic_clearisa (sv=0x0, mg=0xabc3e0) at mg.c:1724
#5  0x0000000000575104 in Perl_magic_setisa (sv=0xab20b0, mg=0xabc3e0) at mg.c:1691
#6  0x000000000056dfa1 in Perl_mg_set (sv=0xab20b0) at mg.c:277
#7  0x00000000005aeacc in Perl_pp_aassign () at pp_hot.c:1422
#8  0x000000000055a245 in Perl_runops_debug () at dump.c:2239
#9  0x00000000004623d3 in S_run_body (oldscope=1) at perl.c:2517
#10 0x00000000004619fe in perl_run (my_perl=0xa9c010) at perl.c:2440
#11 0x000000000041eae0 in main (argc=5, argv=0x7fffffffe5f8, env=0x7fffffffe628)
    at perlmain.c:116
(gdb) f 4
#4  0x000000000057548f in Perl_magic_clearisa (sv=0x0, mg=0xabc3e0) at mg.c:1724
1724        stash = GvSTASH(
(gdb) l
1719            }
1721            return 0;
1722        }
1724        stash = GvSTASH(
1725            (const GV *)mg->mg_obj
1726        );
1728        /* The stash may have been detached from the symbol table, so check its
(gdb) info locals
_gvstash = 0xab2068
stash = 0xab20b0
__PRETTY_FUNCTION__ = "Perl_magic_clearisa"


No reported memory management errors.

**PERL -V**

dcollins@nightshade64:~/perldebug$ ./perl -Ilib -V
Summary of my perl5 (revision 5 version 25 subversion 2) configuration:
  Commit id: c29dfc6a6c45f86648c51f961304254cc3c449b9
    osname=linux, osvers=4.5.0-2-amd64, archname=x86_64-linux-ld
    uname='linux nightshade64 4.5.0-2-amd64 #1 smp debian 4.5.3-2 (2016-05-08) x86_64 gnulinux '
    config_args='-Dusedevel -Dprefix=/usr/local/perl-afl -Dcc=ccache gcc-6.1 -Duselongdouble -Duse64bitall -Doptimize=-g -Uversiononly -Uman1dir -Uman3dir -DDEBUGGING -DPERL_POISON -des'
    hint=recommended, useposix=true, d_sigaction=define
    useithreads=undef, usemultiplicity=undef
    use64bitint=define, use64bitall=define, uselongdouble=define
    usemymalloc=n, bincompat5005=undef
    cc='ccache gcc-6.1', ccflags ='-fwrapv -DDEBUGGING -fno-strict-aliasing -pipe -fstack-protector-strong -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64',
    cppflags='-fwrapv -DDEBUGGING -fno-strict-aliasing -pipe -fstack-protector-strong -I/usr/local/include'
    ccversion='', gccversion='6.1.0', gccosandvers=''
    intsize=4, longsize=8, ptrsize=8, doublesize=8, byteorder=12345678, doublekind=3
    d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=16, longdblkind=3
    ivtype='long', ivsize=8, nvtype='long double', nvsize=16, Off_t='off_t', lseeksize=8
    alignbytes=16, prototype=define
  Linker and Libraries:
    ld='ccache gcc-6.1', ldflags =' -fstack-protector-strong -L/usr/local/lib'
    libpth=/usr/local/lib /usr/local/lib/gcc/x86_64-pc-linux-gnu/6.1.0/include-fixed /usr/include/x86_64-linux-gnu /usr/lib /lib/x86_64-linux-gnu /lib/../lib /usr/lib/x86_64-linux-gnu /usr/lib/../lib /lib
    libs=-lpthread -lnsl -ldl -lm -lcrypt -lutil -lc
    perllibs=-lpthread -lnsl -ldl -lm -lcrypt -lutil -lc, so=so, useshrplib=false, libperl=libperl.a
  Dynamic Linking:
    dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-Wl,-E'
    cccdlflags='-fPIC', lddlflags='-shared -g -L/usr/local/lib -fstack-protector-strong'

Characteristics of this binary (from libperl):
                        USE_64_BIT_ALL USE_64_BIT_INT USE_LARGE_FILES
                        USE_PERLIO USE_PERL_ATOF
  Built under linux
  Compiled at May 26 2016 17:57:37

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