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index() confused by tie side effects #16312

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p5pRT opened this issue Dec 18, 2017 · 1 comment
Open

index() confused by tie side effects #16312

p5pRT opened this issue Dec 18, 2017 · 1 comment

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@p5pRT
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@p5pRT p5pRT commented Dec 18, 2017

Migrated from rt.perl.org#132598 (status was 'new')

Searchable as RT132598$

@p5pRT
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@p5pRT p5pRT commented Dec 18, 2017

From zefram@fysh.org

Created by zefram@fysh.org

Here's an index() operation in which the fetching of the value of one
operand alters the other operand​:

$ perl -lwe 'package T { sub TIESCALAR { bless({}, $_[0]) } sub FETCH { $​::haystack="cc"; $​::haystack = ""; "c" } } $haystack = "ppcp"; chop $haystack; tie $needle, "T"; print index($haystack, $needle)'
1

The answer that this produces is quite surprising. If the search used
the $haystack value that prevailed before the tied fetch, "ppc", index()
would yield 2. If it used the $haystack value that prevailed after the
tied fetch, "", index() would yield -1. For the behaviour to qualify as
sane it ought to be one of those two. Venturing into less sane behaviour,
if the search used the value of $haystack that briefly applies during
the tied fetch before being overwritten, "cc", index() would yield 0.
But what it actually yields, 1, doesn't correspond to any value that
$haystack ever had. This is surely a bug.

What's going on is that pp_index first fetches a pointer to the $haystack
string and the length of that string, before invoking the get magic
on $needle. The get magic overwrites the string and changes the length
of $haystack, but pp_index doesn't notice. It searches in the mutated
$haystack buffer, with the original $haystack length, and that buffer
content is now "\0c\0". Variations on this could obviously read from
a buffer that has been freed.

The chop() in the test case is required to defeat COW. Without it,
pp_index searches the COW-shared buffer, getting a sane result.

Perl Info

Flags:
    category=core
    severity=low

Site configuration information for perl 5.27.6:

Configured by zefram at Tue Nov 21 05:42:59 GMT 2017.

Summary of my perl5 (revision 5 version 27 subversion 6) configuration:
   
  Platform:
    osname=linux
    osvers=3.16.0-4-amd64
    archname=x86_64-linux-thread-multi
    uname='linux barba.rous.org 3.16.0-4-amd64 #1 smp debian 3.16.43-2+deb8u2 (2017-06-26) x86_64 gnulinux '
    config_args='-des -Dprefix=/home/zefram/usr/perl/perl_install/perl-5.27.6-i64-f52 -Duselargefiles -Dusethreads -Uafs -Ud_csh -Uusesfio -Uusenm -Duseshrplib -Dusedevel -Uversiononly -Ui_db'
    hint=recommended
    useposix=true
    d_sigaction=define
    useithreads=define
    usemultiplicity=define
    use64bitint=define
    use64bitall=define
    uselongdouble=undef
    usemymalloc=n
    default_inc_excludes_dot=define
    bincompat5005=undef
  Compiler:
    cc='cc'
    ccflags ='-D_REENTRANT -D_GNU_SOURCE -fwrapv -fno-strict-aliasing -pipe -fstack-protector-strong -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -D_FORTIFY_SOURCE=2'
    optimize='-O2'
    cppflags='-D_REENTRANT -D_GNU_SOURCE -fwrapv -fno-strict-aliasing -pipe -fstack-protector-strong -I/usr/local/include'
    ccversion=''
    gccversion='4.9.2'
    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='double'
    nvsize=8
    Off_t='off_t'
    lseeksize=8
    alignbytes=8
    prototype=define
  Linker and Libraries:
    ld='cc'
    ldflags =' -fstack-protector-strong -L/usr/local/lib'
    libpth=/usr/local/lib /usr/lib/gcc/x86_64-linux-gnu/4.9/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 -ldb -ldl -lm -lcrypt -lutil -lc
    perllibs=-lpthread -lnsl -ldl -lm -lcrypt -lutil -lc
    libc=libc-2.19.so
    so=so
    useshrplib=true
    libperl=libperl.so
    gnulibc_version='2.19'
  Dynamic Linking:
    dlsrc=dl_dlopen.xs
    dlext=so
    d_dlsymun=undef
    ccdlflags='-Wl,-E -Wl,-rpath,/home/zefram/usr/perl/perl_install/perl-5.27.6-i64-f52/lib/5.27.6/x86_64-linux-thread-multi/CORE'
    cccdlflags='-fPIC'
    lddlflags='-shared -O2 -L/usr/local/lib -fstack-protector-strong'



@INC for perl 5.27.6:
    /home/zefram/usr/perl/perl_install/perl-5.27.6-i64-f52/lib/site_perl/5.27.6/x86_64-linux-thread-multi
    /home/zefram/usr/perl/perl_install/perl-5.27.6-i64-f52/lib/site_perl/5.27.6
    /home/zefram/usr/perl/perl_install/perl-5.27.6-i64-f52/lib/5.27.6/x86_64-linux-thread-multi
    /home/zefram/usr/perl/perl_install/perl-5.27.6-i64-f52/lib/5.27.6


Environment for perl 5.27.6:
    HOME=/home/zefram
    LANG (unset)
    LANGUAGE (unset)
    LD_LIBRARY_PATH (unset)
    LOGDIR (unset)
    PATH=/home/zefram/usr/perl/perl_install/perl-5.27.6-i64-f52/bin:/home/zefram/usr/perl/util:/home/zefram/pub/x86_64-unknown-linux-gnu/bin:/home/zefram/pub/common/bin:/usr/bin:/bin:/usr/local/bin:/usr/games
    PERLDOC=-oman
    PERL_BADLANG (unset)
    SHELL=/usr/bin/zsh

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