GNU bug report logs - #62483
echo a | grep -E -w '((()|a)|())*' # does not terminate

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From: arnold <at> skeeve.com
To: dimitry <at> andric.com, arnold <at> skeeve.com
Cc: 62483 <at> debbugs.gnu.org, koen <at> chalmers.se
Subject: bug#62483: echo a | grep -E -w '((()|a)|())*' # does not terminate
Date: Sun, 02 Apr 2023 00:52:02 -0600

Hi.

Dimitry Andric <dimitry <at> andric.com> wrote:

> Yes, it still reproduces when I configure the latest grep using
> --without-included-regex:

I assume you meant --with-included-regex?

If you really used --without-included-regex, that doesn't prove anything... :-)

It's interesting, as gawk uses the same regex, but with different flags.
It might be worth trying to understand which of the syntax bits
is causing this.

Thanks,

Arnold

>
> 1400      for (idx = pmatch[0].rm_so; idx <= pmatch[0].rm_eo ;)
> 1: idx = 0
> (gdb)
> 1402          update_regs (dfa, pmatch, prev_idx_match, cur_node, idx, nmatch);
> 1: idx = 0
> (gdb)
> 1404          if ((idx == pmatch[0].rm_eo && cur_node == mctx->last_node)
> 1: idx = 0
> (gdb)
> 1405              || (fs && re_node_set_contains (&eps_via_nodes, cur_node)))
> 1: idx = 0
> (gdb)
> 1428          cur_node = proceed_next_node (mctx, nmatch, pmatch, prev_idx_match,
> 1: idx = 0
> (gdb)
> 1432          if (__glibc_unlikely (cur_node < 0))
> 1: idx = 0
> (gdb)
> 1400      for (idx = pmatch[0].rm_so; idx <= pmatch[0].rm_eo ;)
> 1: idx = 0
> (gdb)
> 1402          update_regs (dfa, pmatch, prev_idx_match, cur_node, idx, nmatch);
> 1: idx = 0
> (gdb)
> 1404          if ((idx == pmatch[0].rm_eo && cur_node == mctx->last_node)
> 1: idx = 0
> (gdb)
> 1405              || (fs && re_node_set_contains (&eps_via_nodes, cur_node)))
> 1: idx = 0
>
> The endless loop looks the same. grep's regexec.c is mostly the same as
> glibc's, except for the latter having a bunch of #if RE_ENABLE_I18N
> directives added.
>
> -Dimitry
>
> > On 28 Mar 2023, at 08:46, arnold <at> skeeve.com wrote:
> > 
> > This does not reproduce with gawk, even when I force use of the regex
> > matcher.
> > 
> > What happens if grep is built with the included regex files instead of
> > relying on the ones in the local glibc?
> > 
> > Arnold
> > 
> > Dimitry Andric <dimitry <at> andric.com> wrote:
> > 
> >> On 27 Mar 2023, at 11:14, Koen Claessen <koen <at> chalmers.se> wrote:
> >>> 
> >>> Running the command:
> >>> 
> >>> echo a | grep -E -w '((()|a)|())*'
> >>> 
> >>> does not terminate, and uses a LOT of processor time, for all versions of
> >>> grep I have tried.
> >>> 
> >>> This is the smallest case that could be found; simplifying anything in the
> >>> input and/or expression leads to correct behavior.
> >> 
> >> Reproducible with GNU grep 3.7 on Ubuntu 22:
> >> 
> >>    PID USER      PR  NI    VIRT    RES    SHR S  %CPU  %MEM     TIME+ COMMAND
> >>  93938 dim       20   0    9.0m   2.1m   2.0m R 100.0   0.0   0:08.32 grep -E -w ((()|a)|())*
> >> 
> >> It seems that at least on Ubuntu, grep in this mode uses glibc's regexec(3), and it is that implementation which ends up in an endless loop.
> >> 
> >> It loops between lines 1415, 1417 and 1443, but idx and cur_node never change from 0:
> >> 
> >>  1378  static reg_errcode_t
> >>  1379  __attribute_warn_unused_result__
> >>  1380  set_regs (const regex_t *preg, const re_match_context_t *mctx, size_t nmatch,
> >>  1381            regmatch_t *pmatch, bool fl_backtrack)
> >>  1382  {
> >> ...
> >>  1415    for (idx = pmatch[0].rm_so; idx <= pmatch[0].rm_eo ;)
> >>  1416      {
> >>  1417        update_regs (dfa, pmatch, prev_idx_match, cur_node, idx, nmatch);
> >>  1418
> >>  1419        if ((idx == pmatch[0].rm_eo && cur_node == mctx->last_node)
> >>  1420            || (fs && re_node_set_contains (&eps_via_nodes, cur_node)))
> >> ...
> >>  1442        /* Proceed to next node.  */
> >>  1443        cur_node = proceed_next_node (mctx, nmatch, pmatch, prev_idx_match,
> >>  1444                                      &idx, cur_node,
> >>  1445                                      &eps_via_nodes, fs);
> >>  1446
> >>  1447        if (__glibc_unlikely (cur_node < 0))
> >> ...
> >>  1465          }
> >>  1466      }
> >> 
> >> -Dimitry
> >> 
> >> P.S.: Interestingly this does not reproduce with BSD grep, which returns immediately with "a".
> >> 
>

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