Re: question about regex

[Paul Eggert <eggert at cs dot ucla dot edu>]

On 1/2/20 8:16 AM, Tim Rühsen wrote:
> Meanwhile grep (or libc) seems to exit gracefully:

Yes, there's no core dump if the operating system supports stack 
overflow detection that grep can use. The problem occurs only on OSes 
that don't do that, or on apps that don't try to detect stack overflow 
and simply dump core (or worse).

On 1/2/20 2:54 AM, liqingqing wrote:

> do we have any plan or good ways to fix up the bug as below

The best way would be to fix bug#24269, i.e., fix the glibc regex code 
so that it doesn't blow the stack. If you could write a patch for this 
bug (something that doesn't hurt performance for ordinary regexps), that 
would be welcome.

For that particular test case, you can use an OS that does proper stack 
overflow checking that grep can use.

PS. The next version of the grep manual is planned to nearly wash its 
hands of the matter. Here's the current draft:

----

Back-references can greatly slow down matching, as they can generate
exponentially many matching possibilities that can consume both time
and memory to explore.  Also, the POSIX specification for
back-references is at times unclear.  Furthermore, many regular
expression implementations have back-reference bugs that can cause
programs to return incorrect answers or even crash, and fixing these
bugs has often been low-priority: for example, as of 2020 the
@url{https://sourceware.org/bugzilla/,GNU C library bug database}
contained back-reference bugs
@url{https://sourceware.org/bugzilla/show_bug.cgi?id=52,,52},
@url{https://sourceware.org/bugzilla/show_bug.cgi?id=10844,,10844},
@url{https://sourceware.org/bugzilla/show_bug.cgi?id=11053,,11053},
@url{https://sourceware.org/bugzilla/show_bug.cgi?id=24269,,24269}
and @url{https://sourceware.org/bugzilla/show_bug.cgi?id=25322,,25322},
with little sign of forthcoming fixes.  Luckily,
back-references are rarely useful and it should be little trouble to
avoid them in practical applications.