user/dev discussion of public-inbox itself
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From: Eric Wong <e@80x24.org>
To: meta@public-inbox.org
Subject: [PATCH 18/21] ds: guard against stack-not-refcounted quirk of Perl 5
Date: Sun, 31 Jan 2021 22:28:30 -1000	[thread overview]
Message-ID: <20210201082833.3293-19-e@80x24.org> (raw)
In-Reply-To: <20210201082833.3293-1-e@80x24.org>

The Perl 5 stack is weakly-referenced for performance reasons.
This means it's possible for items in the stack to be freed
while executing further down the stack.

In lei (and perhaps public-facing read-only daemons in the
future), we'll fork and call PublicInbox::DS->Reset in the child
process.  This causes %DescriptorMap to be clobbered, allowing
the $DescriptorMap{$fd} arg to be freed inside the child
process.

When Carp::confess or Carp::longmess is called to generate a
backtrace, it may access the @DB::args array.  This array access
is not protected by reference counting and is known to cause
segfaults and other weird errors.

While the caller of an unnecessary Carp::confess may be
eliminated in a future commit, we can't guarantee our
dependencies will be free of @DB::args access attempts
in the future.

So guard against this Perl 5 quirmk by defensively bumping the
refcount of any object we call ->event_step on.

cf. https://rt.perl.org/Public/Bug/Display.html?id=131046
    https://github.com/Perl/perl5/issues/15928
---
 lib/PublicInbox/DS.pm | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/lib/PublicInbox/DS.pm b/lib/PublicInbox/DS.pm
index 40994fd4..2d312f0a 100644
--- a/lib/PublicInbox/DS.pm
+++ b/lib/PublicInbox/DS.pm
@@ -271,6 +271,7 @@ sub EventLoop {
     $Epoll //= _InitPoller();
     local $in_loop = 1;
     my @events;
+    my $obj; # guard stack-not-refcounted w/ Carp + @DB::args
     do {
         my $timeout = RunTimers();
 
@@ -281,7 +282,8 @@ sub EventLoop {
             # that ones in the front triggered unregister-interest actions.  if we
             # can't find the %sock entry, it's because we're no longer interested
             # in that event.
-            $DescriptorMap{$fd}->event_step;
+            $obj = $DescriptorMap{$fd};
+            $obj->event_step;
         }
     } while (PostEventLoop());
     _run_later();

  parent reply	other threads:[~2021-02-01  8:28 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-02-01  8:28 [PATCH 00/21] lei2mail worker segfault finally fixed Eric Wong
2021-02-01  8:28 ` [PATCH 01/21] lei: more consistent dedupe and ovv_buf init Eric Wong
2021-02-01  8:28 ` [PATCH 02/21] ipc: switch wq to use the event loop Eric Wong
2021-02-01  8:28 ` [PATCH 03/21] lei: remove per-child SIG{__WARN__} Eric Wong
2021-02-01  8:28 ` [PATCH 04/21] lei: remove SIGPIPE handler Eric Wong
2021-02-01  8:28 ` [PATCH 05/21] ipc: more helpful ETOOMANYREFS error messages Eric Wong
2021-02-01  8:28 ` [PATCH 06/21] lei: remove syslog dependency Eric Wong
2021-02-01  8:28 ` [PATCH 07/21] sharedkv: release {dbh} before rmtree Eric Wong
2021-02-01  8:28 ` [PATCH 08/21] lei: keep $lei around until workers are reaped Eric Wong
2021-02-01  8:28 ` [PATCH 09/21] lei_dedupe: use Digest::SHA Eric Wong
2021-02-01  8:28 ` [PATCH 10/21] lei_xsearch: load PublicInbox::Smsg Eric Wong
2021-02-01  8:28 ` [PATCH 11/21] lei: deep clone {ovv} for l2m workers Eric Wong
2021-02-01  8:28 ` [PATCH 12/21] sharedkv: lock and explicitly disconnect {dbh} Eric Wong
2021-02-01  8:28 ` [PATCH 13/21] lei: increase initial timeout Eric Wong
2021-02-01  8:28 ` [PATCH 14/21] sharedkv: use lock_for_scope_fast Eric Wong
2021-02-01  8:28 ` [PATCH 15/21] lei_to_mail: reduce spew on Maildir removal Eric Wong
2021-02-01  8:28 ` [PATCH 16/21] sharedkv: do not set cache_size by default Eric Wong
2021-02-01  8:28 ` [PATCH 17/21] import: reap git-config(1) synchronously Eric Wong
2021-02-01  8:28 ` Eric Wong [this message]
2021-02-01  9:07   ` Perl debug patches used to track down source of segfault Eric Wong
2021-02-01  8:28 ` [PATCH 19/21] ds: next_tick: avoid $_ in top-level loop iterator Eric Wong
2021-02-01  8:28 ` [PATCH 20/21] lei: avoid ETOOMANYREFS, cleanup imports Eric Wong
2021-02-01  8:28 ` [PATCH 21/21] doc: note optional BSD::Resource use Eric Wong

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