[pve-devel] [PATCH kernel] cherry-pick fix to surpress faulty segfault logging

[Fiona Ebner]

While there is no actual issue, users are still nervous about the
faulty logging [0]. It might take a while until the fix comes in via
upstream, so just pick it up manually.

[0]: https://forum.proxmox.com/threads/130628/post-583864

Signed-off-by: Fiona Ebner <f.ebner at proxmox.com>
---
 ...ault-logging-if-fatal-signal-already.patch | 67 +++++++++++++++++++
 1 file changed, 67 insertions(+)
 create mode 100644 patches/kernel/0037-mm-suppress-mm-fault-logging-if-fatal-signal-already.patch

diff --git a/patches/kernel/0037-mm-suppress-mm-fault-logging-if-fatal-signal-already.patch b/patches/kernel/0037-mm-suppress-mm-fault-logging-if-fatal-signal-already.patch
new file mode 100644
index 0000000..769811b
--- /dev/null
+++ b/patches/kernel/0037-mm-suppress-mm-fault-logging-if-fatal-signal-already.patch
@@ -0,0 +1,67 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Linus Torvalds <torvalds at linux-foundation.org>
+Date: Tue, 25 Jul 2023 09:38:32 -0700
+Subject: [PATCH] mm: suppress mm fault logging if fatal signal already pending
+
+Commit eda0047296a1 ("mm: make the page fault mmap locking killable")
+intentionally made it much easier to trigger the "page fault fails
+because a fatal signal is pending" situation, by having the mmap locking
+fail early in that case.
+
+We have long aborted page faults in other fatal cases when the actual IO
+for a page is interrupted by SIGKILL - which is particularly useful for
+the traditional case of NFS hanging due to network issues, but local
+filesystems could cause it too if you happened to get the SIGKILL while
+waiting for a page to be faulted in (eg lock_folio_maybe_drop_mmap()).
+
+So aborting the page fault wasn't a new condition - but it now triggers
+earlier, before we even get to 'handle_mm_fault()'.  And as a result the
+error doesn't go through our 'fault_signal_pending()' logic, and doesn't
+get filtered away there.
+
+Normally you'd never even notice, because if a fatal signal is pending,
+the new SIGSEGV we send ends up being ignored anyway.
+
+But it turns out that there is one very noticeable exception: if you
+enable 'show_unhandled_signals', the aborted page fault will be logged
+in the kernel messages, and you'll get a scary line looking something
+like this in your logs:
+
+  pverados[2183248]: segfault at 55e5a00f9ae0 ip 000055e5a00f9ae0 sp 00007ffc0720bea8 error 14 in perl[55e5a00d4000+195000] likely on CPU 10 (core 4, socket 0)
+
+which is rather misleading.  It's not really a segfault at all, it's
+just "the thread was killed before the page fault completed, so we
+aborted the page fault".
+
+Fix this by just making it clear that a pending fatal signal means that
+any new signal coming in after that is implicitly handled.  This will
+avoid the misleading logging, since now the signal isn't 'unhandled' any
+more.
+
+Reported-and-tested-by: Fiona Ebner <f.ebner at proxmox.com>
+Tested-by: Thomas Lamprecht <t.lamprecht at proxmox.com>
+Link: https://lore.kernel.org/lkml/8d063a26-43f5-0bb7-3203-c6a04dc159f8@proxmox.com/
+Acked-by: Oleg Nesterov <oleg at redhat.com>
+Fixes: eda0047296a1 ("mm: make the page fault mmap locking killable")
+Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
+(cherry-picked from commit 5f0bc0b042fc77ff70e14c790abdec960cde4ec1)
+Signed-off-by: Fiona Ebner <f.ebner at proxmox.com>
+---
+ kernel/signal.c | 4 ++++
+ 1 file changed, 4 insertions(+)
+
+diff --git a/kernel/signal.c b/kernel/signal.c
+index ae26da61c4d9..060f834e9c1a 100644
+--- a/kernel/signal.c
++++ b/kernel/signal.c
+@@ -561,6 +561,10 @@ bool unhandled_signal(struct task_struct *tsk, int sig)
+ 	if (handler != SIG_IGN && handler != SIG_DFL)
+ 		return false;
+ 
++	/* If dying, we handle all new signals by ignoring them */
++	if (fatal_signal_pending(tsk))
++		return false;
++
+ 	/* if ptraced, let the tracer determine */
+ 	return !tsk->ptrace;
+ }
-- 
2.39.2