op.c - Restrict nested eval/BEGIN blocks to a user controllable maximum

Nested BEGIN blocks can cause us to segfault by exhausting
the C stack. Eg:

    perl -le'sub f { eval "BEGIN { f() }" } f()'

will segfault. This adds a new interpreter var PL_eval_begin_nest_depth
to keep track of how many layer of eval/BEGIN we have seen, and a new
reserved variable called ${^MAX_NESTED_EVAL_BEGIN_BLOCKS} which can be
used to raise or lower the limit. When set to 0 it blocks BEGIN entirely,
which might be useful from time to time.

This fixes https://github.com/Perl/perl5/issues/20176

1 files changed, 10 insertions, 0 deletions

diff --git a/pod/perlfunc.pod b/pod/perlfunc.pod
index fa16cbd69c..b723cbbcb1 100644
--- a/pod/perlfunc.pod
+++ b/pod/perlfunc.pod
@@ -2237,6 +2237,16 @@ context of the current Perl program, any outer lexical variables are
 visible to it, and any package variable settings or subroutine and
 format definitions remain afterwards.
 
+Note that when C<BEGIN {}> blocks are embedded inside of an eval block
+the contents of the block will be executed immediately and before the rest
+of the eval code is executed. You can disable this entirely by
+
+   local ${^MAX_NESTED_EVAL_BEGIN_BLOCKS} = 0;
+   eval $string;
+
+which will cause any embedded C<BEGIN> blocks in C<$string> to throw an
+exception.
+
 =over 4
 
 =item Under the L<C<"unicode_eval"> feature|feature/The 'unicode_eval' and 'evalbytes' features>