linux-kernel - [PATCH] proc: fix test for "vsyscall=xonly" boot option

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Message-ID: <Ys2Hi3Ps933B6IsE@localhost.localdomain>
Date:   Tue, 12 Jul 2022 17:39:07 +0300
From:   Alexey Dobriyan <adobriyan@...il.com>
To:     akpm@...ux-foundation.org
Cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kselftest@...r.kernel.org, dylanbhatch@...gle.com,
        skhan@...uxfoundation.org
Subject: [PATCH] proc: fix test for "vsyscall=xonly" boot option

Booting with vsyscall=xonly results in the "--xp" vsyscall VMA:

	ffffffffff600000-ffffffffff601000 --xp ... [vsyscall]\n

Test does read from fixed vsyscall address to determine if kernel
supports vsyscall page but it doesn't work with vsyscall=xonly
because, well, vsyscall page is execute only.

Fix test by trying to execute from the first byte of the page
(which contains gettimeofday() stub). This should work because vsyscall
entry points themselves have stable addresses by design.

	Alexey, avoiding parsing .config, /proc/config.gz and
	/proc/cmdline at all costs.

Signed-off-by: Alexey Dobriyan <adobriyan@...il.com>
---

	I'm not sure who reported what, so please add tested-by and
	reported-by lines.

 tools/testing/selftests/proc/proc-pid-vm.c |   75 ++++++++++++++++++++++++++---
 1 file changed, 68 insertions(+), 7 deletions(-)

--- a/tools/testing/selftests/proc/proc-pid-vm.c
+++ b/tools/testing/selftests/proc/proc-pid-vm.c
@@ -211,10 +211,19 @@ static int make_exe(const uint8_t *payload, size_t len)
 }
 #endif
 
-static bool g_vsyscall = false;
+/*
+ * 0: vsyscall VMA doesn't exist	vsyscall=none
+ * 1: vsyscall VMA is r-xp		vsyscall=emulate
+ * 2: vsyscall VMA is --xp		vsyscall=xonly
+ */
+static int g_vsyscall;
+static const char *str_vsyscall;
 
-static const char str_vsyscall[] =
+static const char str_vsyscall_0[] = "";
+static const char str_vsyscall_1[] =
 "ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0                  [vsyscall]\n";
+static const char str_vsyscall_2[] =
+"ffffffffff600000-ffffffffff601000 --xp 00000000 00:00 0                  [vsyscall]\n";
 
 #ifdef __x86_64__
 static void sigaction_SIGSEGV(int _, siginfo_t *__, void *___)
@@ -223,13 +232,47 @@ static void sigaction_SIGSEGV(int _, siginfo_t *__, void *___)
 }
 
 /*
- * vsyscall page can't be unmapped, probe it with memory load.
+ * vsyscall page can't be unmapped, probe it directly.
  */
 static void vsyscall(void)
 {
 	pid_t pid;
 	int wstatus;
 
+	pid = fork();
+	if (pid < 0) {
+		fprintf(stderr, "fork, errno %d\n", errno);
+		exit(1);
+	}
+	if (pid == 0) {
+		struct rlimit rlim = {0, 0};
+		(void)setrlimit(RLIMIT_CORE, &rlim);
+
+		/* Hide "segfault at ffffffffff600000" messages. */
+		struct sigaction act;
+		memset(&act, 0, sizeof(struct sigaction));
+		act.sa_flags = SA_SIGINFO;
+		act.sa_sigaction = sigaction_SIGSEGV;
+		(void)sigaction(SIGSEGV, &act, NULL);
+
+		/* gettimeofday(NULL, NULL); */
+		asm volatile (
+			"call %P0"
+			:
+			: "i" (0xffffffffff600000), "D" (NULL), "S" (NULL)
+			: "rax"
+		);
+		exit(0);
+	}
+	waitpid(pid, &wstatus, 0);
+	if (WIFEXITED(wstatus) && WEXITSTATUS(wstatus) == 0) {
+		/* vsyscall page exists and is executable. */
+	} else {
+		/* vsyscall page doesn't exist. */
+		g_vsyscall = 0;
+		return;
+	}
+
 	pid = fork();
 	if (pid < 0) {
 		fprintf(stderr, "fork, errno %d\n", errno);
@@ -251,8 +294,13 @@ static void vsyscall(void)
 	}
 	waitpid(pid, &wstatus, 0);
 	if (WIFEXITED(wstatus) && WEXITSTATUS(wstatus) == 0) {
-		g_vsyscall = true;
+		/* vsyscall page is readable and executable. */
+		g_vsyscall = 1;
+		return;
 	}
+
+	/* vsyscall page is executable but unreadable. */
+	g_vsyscall = 2;
 }
 
 int main(void)
@@ -261,6 +309,19 @@ int main(void)
 	int exec_fd;
 
 	vsyscall();
+	switch (g_vsyscall) {
+	case 0:
+		str_vsyscall = str_vsyscall_0;
+		break;
+	case 1:
+		str_vsyscall = str_vsyscall_1;
+		break;
+	case 2:
+		str_vsyscall = str_vsyscall_2;
+		break;
+	default:
+		abort();
+	}
 
 	atexit(ate);
 
@@ -314,7 +375,7 @@ int main(void)
 
 	/* Test /proc/$PID/maps */
 	{
-		const size_t len = strlen(buf0) + (g_vsyscall ? strlen(str_vsyscall) : 0);
+		const size_t len = strlen(buf0) + strlen(str_vsyscall);
 		char buf[256];
 		ssize_t rv;
 		int fd;
@@ -327,7 +388,7 @@ int main(void)
 		rv = read(fd, buf, sizeof(buf));
 		assert(rv == len);
 		assert(memcmp(buf, buf0, strlen(buf0)) == 0);
-		if (g_vsyscall) {
+		if (g_vsyscall > 0) {
 			assert(memcmp(buf + strlen(buf0), str_vsyscall, strlen(str_vsyscall)) == 0);
 		}
 	}
@@ -374,7 +435,7 @@ int main(void)
 			assert(memmem(buf, rv, S[i], strlen(S[i])));
 		}
 
-		if (g_vsyscall) {
+		if (g_vsyscall > 0) {
 			assert(memmem(buf, rv, str_vsyscall, strlen(str_vsyscall)));
 		}
 	}