From 81b3a0a34145ee6c855f50c8035728f76d63c3f0 Mon Sep 17 00:00:00 2001
From: Konstantin Belousov <kib@FreeBSD.org>
Date: Sun, 10 Jan 2021 05:05:42 +0200
Subject: [PATCH] libc: implement rtld_get_stack_prot() for real

which makes stack prot correct for non-main threads created by binaries
with statically linked libthr.

Cache result, but do not engage into the full double-checked locking,
since calculation of the return value is idempotent.

PR:	252549
Reported and reviewed by:	emaste
MFC after:	1 week
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D28075
---
 lib/libc/gen/dlfcn.c | 26 +++++++++++++++++++++++++-
 1 file changed, 25 insertions(+), 1 deletion(-)

diff --git a/lib/libc/gen/dlfcn.c b/lib/libc/gen/dlfcn.c
index f7f162753b4b..395a6d9402e8 100644
--- a/lib/libc/gen/dlfcn.c
+++ b/lib/libc/gen/dlfcn.c
@@ -34,7 +34,9 @@ __FBSDID("$FreeBSD$");
 /*
  * Linkage to services provided by the dynamic linker.
  */
+#include <sys/types.h>
 #include <sys/mman.h>
+#include <machine/atomic.h>
 #include <dlfcn.h>
 #include <link.h>
 #include <stddef.h>
@@ -256,8 +258,30 @@ _rtld_addr_phdr(const void *addr __unused,
 int
 _rtld_get_stack_prot(void)
 {
+#ifndef IN_LIBDL
+	unsigned i;
+	int r;
+	static int ret;
 
-	return (PROT_EXEC | PROT_READ | PROT_WRITE);
+	r = atomic_load_int(&ret);
+	if (r != 0)
+		return (r);
+
+	_once(&dl_phdr_info_once, dl_init_phdr_info);
+	r = PROT_EXEC | PROT_READ | PROT_WRITE;
+	for (i = 0; i < phdr_info.dlpi_phnum; i++) {
+		if (phdr_info.dlpi_phdr[i].p_type != PT_GNU_STACK)
+			continue;
+		r = PROT_READ | PROT_WRITE;
+		if ((phdr_info.dlpi_phdr[i].p_flags & PF_X) != 0)
+			r |= PROT_EXEC;
+		break;
+	}
+	atomic_store_int(&ret, r);
+	return (r);
+#else
+	return (0);
+#endif
 }
 
 #pragma weak _rtld_is_dlopened