sed segfault after commit 9c47c43e0736 with FEATURE_CLEAN_UP

[Thomas De Schampheleire]

On Tue, Sep 25, 2018, 12:53 Denys Vlasenko <vda.linux at googlemail.com> wrote:

> On Mon, Sep 24, 2018 at 4:36 PM Thomas De Schampheleire
> <patrickdepinguin at gmail.com> wrote:
> > If FEATURE_CLEAN_UP is enabled, following reproduction test case
> > causes a segfault:
> >
> > +testing "sed uses previous regexp test2" \
> > +       "sed -n '/foo/ { //p }'" \
> > +       "" \
> > +       "" \
> > +       "q\nw\ne\nr\n"
> > +
> >
> > (note: the test case passes because the output is correct; it seems
> > the test suite does not catch segfaults)
> >
> > The issue is bisected to commit 9c47c43e0736 which adds support for
> > '//' to indicate the previous regex. The change in question is:
> >
> > -               temp = copy_parsing_escapes(pos, next);
> > -               *regex = xzalloc(sizeof(regex_t));
> > -               xregcomp(*regex, temp, G.regex_type);
> > -               free(temp);
> > +               if (next != 0) {
> > +                       temp = copy_parsing_escapes(pos, next);
> > +                       G.previous_regex_ptr = *regex =
> > xzalloc(sizeof(regex_t));
> > +                       xregcomp(*regex, temp, G.regex_type);
> > +                       free(temp);
> > +               } else {
> > +                       *regex = G.previous_regex_ptr;
> > +                       if (!G.previous_regex_ptr)
> > +                               bb_error_msg_and_die("no previous
> regexp");
> > +               }
> >
> >
> > In the original code, *regex was always a newly allocated pointer.
> > In the new code, there are cases where *regex (mapping to
> > sed_cmd.beg_match in the caller) is not a newly allocated pointer, but
> > the same as the previously allocated one.
> >
> > When sed finishes, and sed_free_and_close_stuff() is called, there is
> > an iteration over the different sed_cmd structures. Following code:
> >
> >         if (sed_cmd->beg_match) {
> >             regfree(sed_cmd->beg_match);
> >             free(sed_cmd->beg_match);
> >         }
> >
> > will work fine in the first iteration that touches the repeated
> > pointer, but will fail in 'regfree' on the next iteration, where the
> > beg_match pointer points to the same value as was already freed.
> >
> > Backtrace is:
> > Program received signal SIGSEGV, Segmentation fault.
> > free_token (node=0x198) at regcomp.c:3808
> > 3808    regcomp.c: No such file or directory.
> > (gdb) bt
> > #0  free_token (node=0x198) at regcomp.c:3808
> > #1  0xf6fe474c in free_dfa_content (dfa=0x712220) at regcomp.c:598
> > #2  0xf6ff1200 in __regfree (preg=0x712068) at regcomp.c:647
> > #3  0x000d53dc in sed_free_and_close_stuff () at editors/sed.c:184
> > #4  0xf6f6be94 in __run_exit_handlers (status=0x0, listp=0xf70744b4
> > <__exit_funcs>,
> >     run_list_atexit=run_list_atexit at entry=0x1) at exit.c:82
> > #5  0xf6f6bee8 in __GI_exit (status=<optimized out>) at exit.c:104
> > #6  0x0001a194 in xfunc_die () at libbb/xfunc_die.c:20
> > #7  0x000196b0 in run_applet_no_and_exit (applet_no=0xa9,
> > name=0xfff7ce6a "sed", argv=0xfff7bf14)
> >     at libbb/appletlib.c:952
> > #8  0x00019718 in run_applet_and_exit (name=0xfff7ce6a "sed",
> > argv=0xfff7bf14) at libbb/appletlib.c:968
> > #9  0x000197fc in main (argc=0x4, argv=0xfff7bf14) at
> libbb/appletlib.c:1076
> >
> >
> > I'm not sure how to fix this:
> > * either keep a list in sed_free_and_close_stuff of pointers already
> freed
> > * or create a separate list of pointers-to-free when allocating
> > pointers rather than assuming that all values in beg_match need to be
> > freed
> > * or add a flag to indicate this special case in sed_cmd, which would
> > need some more communication between get_address and its caller.
> > * something else?
>
> How about "don't free them at all"?
>

Doesn't this contradict with the purpose of FEATURE_CLEAN_UP?

>
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