nptl: Install SIGSETXID handler with SA_ONSTACK [BZ #27914]

glibc-sigsetxid-sa_onstack.patch

@@ -0,0 +1,36 @@
+Emergency backport of this change prior to upstream acceptance:
+
+Author: Florian Weimer <fweimer@redhat.com>
+Date:   Wed May 26 07:13:00 2021 +0200
+
+    nptl: Install SIGSETXID handler with SA_ONSTACK [BZ #27914]
+    
+    The signal is sent to all threads, some of which may have switched
+    to very small stacks.  If they have also installed an alternate
+    signal stack, SA_ONSTACK makes this work.  The Go runtime needs this:
+    
+      runtime: C.setuid/C.setgid smashes Go stack
+      <https://github.com/golang/go/issues/9400>
+
+    Doing this for SIGCANCEL is less obviously beneficial and needs further
+    testing.
+
+diff --git a/nptl/pthread_create.c b/nptl/pthread_create.c
+index 5680687efe7089da..b7073a828549d24c 100644
+--- a/nptl/pthread_create.c
++++ b/nptl/pthread_create.c
+@@ -83,9 +83,12 @@ late_init (void)
+       (void) __libc_sigaction (SIGCANCEL, &sa, NULL);
+     }
+ 
+-  /* Install the handle to change the threads' uid/gid.  */
++  /* Install the handle to change the threads' uid/gid.  Use
++     SA_ONSTACK because the signal may be sent to threads that are
++     running with custom stacks.  (This is less likely for
++     SIGCANCEL.)  */
+   sa.sa_sigaction = __nptl_setxid_sighandler;
+-  sa.sa_flags = SA_SIGINFO | SA_RESTART;
++  sa.sa_flags = SA_ONSTACK | SA_SIGINFO | SA_RESTART;
+   (void) __libc_sigaction (SIGSETXID, &sa, NULL);
+ 
+   /* The parent process might have left the signals blocked.  Just in

glibc.spec

@@ -97,7 +97,7 @@
 Summary: The GNU libc libraries
 Name: glibc
 Version: %{glibcversion}
-Release: 9%{?dist}
+Release: 10%{?dist}
 
 # In general, GPLv2+ is used by programs, LGPLv2+ is used for
 # libraries.
@@ -170,6 +170,7 @@ Patch23: glibc-python3.patch
 Patch29: glibc-fedora-nsswitch.patch
 Patch30: glibc-deprecated-selinux-makedb.patch
 Patch31: glibc-deprecated-selinux-nscd.patch
+Patch32: glibc-sigsetxid-sa_onstack.patch
 
 ##############################################################################
 # Continued list of core "glibc" package information:
@@ -2112,6 +2113,9 @@ fi
 %files -f compat-libpthread-nonshared.filelist -n compat-libpthread-nonshared
 
 %changelog
+* Wed May 26 2021 Florian Weimer <fweimer@redhat.com> - 2.33.9000-10
+- nptl: Install SIGSETXID handler with SA_ONSTACK [BZ #27914]
+
 * Tue May 25 2021 Florian Weimer <fweimer@redhat.com> - 2.33.9000-9
 - Auto-sync with upstream branch master,
   commit ac0353af81a23535f517586a5d04427120a157ac.