[PATCH v5 0/4] make sigaltstack() compatible with swapcontext()

@@ -19,6 +19,7 @@ TARGETS += powerpc
 TARGETS += pstore
 TARGETS += ptrace
 TARGETS += seccomp
+TARGETS += sigaltstack
 TARGETS += size
 TARGETS += static_keys
 TARGETS += sysctl

@@ -0,0 +1,8 @@
+CFLAGS = -Wall
+BINARIES = sas
+all: $(BINARIES)
+
+include ../lib.mk
+
+clean:
+	rm -rf $(BINARIES)

@@ -0,0 +1,156 @@
+/*
+ * Stas Sergeev <stsp@users.sourceforge.net>
+ *
+ * test sigaltstack(SS_ONSTACK | SS_AUTODISARM)
+ * If that succeeds, then swapcontext() can be used inside sighandler safely.
+ *
+ */
+
+#define _GNU_SOURCE
+#include <signal.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/mman.h>
+#include <ucontext.h>
+#include <alloca.h>
+#include <string.h>
+#include <assert.h>
+
+#ifndef SS_AUTODISARM
+#define SS_AUTODISARM  (1 << 4)
+#endif
+
+static void *sstack, *ustack;
+static ucontext_t uc, sc;
+static const char *msg = "[OK]\tStack preserved";
+static const char *msg2 = "[FAIL]\tStack corrupted";
+struct stk_data {
+	char msg[128];
+	int flag;
+};
+
+void my_usr1(int sig, siginfo_t *si, void *u)
+{
+	char *aa;
+	int err;
+	stack_t stk;
+	struct stk_data *p;
+
+	register unsigned long sp asm("sp");
+
+	if (sp < (unsigned long)sstack ||
+			sp >= (unsigned long)sstack + SIGSTKSZ) {
+		printf("[FAIL]\tSP is not on sigaltstack\n");
+		exit(EXIT_FAILURE);
+	}
+	/* put some data on stack. other sighandler will try to overwrite it */
+	aa = alloca(1024);
+	assert(aa);
+	p = (struct stk_data *)(aa + 512);
+	strcpy(p->msg, msg);
+	p->flag = 1;
+	printf("[RUN]\tsignal USR1\n");
+	err = sigaltstack(NULL, &stk);
+	if (err) {
+		perror("[FAIL]\tsigaltstack()");
+		exit(EXIT_FAILURE);
+	}
+	if (stk.ss_flags != SS_DISABLE)
+		printf("[FAIL]\tss_flags=%i, should be SS_DISABLE\n",
+				stk.ss_flags);
+	else
+		printf("[OK]\tsigaltstack is disabled in sighandler\n");
+	swapcontext(&sc, &uc);
+	printf("%s\n", p->msg);
+	if (!p->flag) {
+		printf("[RUN]\tAborting\n");
+		exit(EXIT_FAILURE);
+	}
+}
+
+void my_usr2(int sig, siginfo_t *si, void *u)
+{
+	char *aa;
+	struct stk_data *p;
+
+	printf("[RUN]\tsignal USR2\n");
+	aa = alloca(1024);
+	/* dont run valgrind on this */
+	/* try to find the data stored by previous sighandler */
+	p = memmem(aa, 1024, msg, strlen(msg));
+	if (p) {
+		printf("[FAIL]\tsigaltstack re-used\n");
+		/* corrupt the data */
+		strcpy(p->msg, msg2);
+		/* tell other sighandler that his data is corrupted */
+		p->flag = 0;
+	}
+}
+
+static void switch_fn(void)
+{
+	printf("[RUN]\tswitched to user ctx\n");
+	raise(SIGUSR2);
+	setcontext(&sc);
+}
+
+int main(void)
+{
+	struct sigaction act;
+	stack_t stk;
+	int err;
+
+	sigemptyset(&act.sa_mask);
+	act.sa_flags = SA_ONSTACK | SA_SIGINFO;
+	act.sa_sigaction = my_usr1;
+	sigaction(SIGUSR1, &act, NULL);
+	act.sa_sigaction = my_usr2;
+	sigaction(SIGUSR2, &act, NULL);
+	sstack = mmap(NULL, SIGSTKSZ, PROT_READ | PROT_WRITE,
+		      MAP_PRIVATE | MAP_ANONYMOUS | MAP_STACK, -1, 0);
+	if (sstack == MAP_FAILED) {
+		perror("mmap()");
+		return EXIT_FAILURE;
+	}
+	stk.ss_sp = sstack;
+	stk.ss_size = SIGSTKSZ;
+	stk.ss_flags = SS_ONSTACK | SS_AUTODISARM;
+	err = sigaltstack(&stk, NULL);
+	if (err) {
+		perror("[FAIL]\tsigaltstack(SS_ONSTACK | SS_AUTODISARM)");
+		stk.ss_flags = SS_ONSTACK;
+	}
+	err = sigaltstack(&stk, NULL);
+	if (err) {
+		perror("[FAIL]\tsigaltstack(SS_ONSTACK)");
+		return EXIT_FAILURE;
+	}
+
+	ustack = mmap(NULL, SIGSTKSZ, PROT_READ | PROT_WRITE,
+		      MAP_PRIVATE | MAP_ANONYMOUS | MAP_STACK, -1, 0);
+	if (ustack == MAP_FAILED) {
+		perror("mmap()");
+		return EXIT_FAILURE;
+	}
+	getcontext(&uc);
+	uc.uc_link = NULL;
+	uc.uc_stack.ss_sp = ustack;
+	uc.uc_stack.ss_size = SIGSTKSZ;
+	makecontext(&uc, switch_fn, 0);
+	raise(SIGUSR1);
+
+	err = sigaltstack(NULL, &stk);
+	if (err) {
+		perror("[FAIL]\tsigaltstack()");
+		exit(EXIT_FAILURE);
+	}
+	if (stk.ss_flags != 0) {
+		printf("[FAIL]\tss_flags=%i, should be 0\n",
+				stk.ss_flags);
+		exit(EXIT_FAILURE);
+	}
+	printf("[OK]\tsigaltstack is enabled after signal\n");
+
+	printf("[OK]\tTest passed\n");
+	return 0;
+}

@@ -1587,6 +1587,7 @@ struct task_struct {
 
 	unsigned long sas_ss_sp;
 	size_t sas_ss_size;
+	unsigned sas_ss_flags;
 
 	struct callback_head *task_works;
 

@@ -2573,6 +2574,13 @@ static inline int sas_ss_flags(unsigned long sp)
 	return on_sig_stack(sp) ? SS_ONSTACK : 0;
 }
 
+static inline void sas_ss_reset(struct task_struct *p)
+{
+	p->sas_ss_sp = 0;
+	p->sas_ss_size = 0;
+	p->sas_ss_flags = SS_DISABLE;
+}
+
 static inline unsigned long sigsp(unsigned long sp, struct ksignal *ksig)
 {
 	if (unlikely((ksig->ka.sa.sa_flags & SA_ONSTACK)) && ! sas_ss_flags(sp))

@@ -432,8 +432,10 @@ int __save_altstack(stack_t __user *, unsigned long);
 	stack_t __user *__uss = uss; \
 	struct task_struct *t = current; \
 	put_user_ex((void __user *)t->sas_ss_sp, &__uss->ss_sp); \
-	put_user_ex(sas_ss_flags(sp), &__uss->ss_flags); \
+	put_user_ex(t->sas_ss_flags, &__uss->ss_flags); \
 	put_user_ex(t->sas_ss_size, &__uss->ss_size); \
+	if (t->sas_ss_flags & SS_AUTODISARM) \
+		sas_ss_reset(t); \
 } while (0);
 
 #ifdef CONFIG_PROC_FS

@@ -7,7 +7,9 @@
 #define SS_ONSTACK	1
 #define SS_DISABLE	2
 
+/* bit-flags */
+#define SS_AUTODISARM	(1 << 4)	/* disable sas during sighandling */
 /* mask for all SS_xxx flags */
-#define SS_FLAG_BITS	0
+#define SS_FLAG_BITS	SS_AUTODISARM
 
 #endif /* _UAPI_LINUX_SIGNAL_H */

@@ -1483,7 +1483,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 	 * sigaltstack should be cleared when sharing the same VM
 	 */
 	if ((clone_flags & (CLONE_VM|CLONE_VFORK)) == CLONE_VM)
-		p->sas_ss_sp = p->sas_ss_size = 0;
+		sas_ss_reset(p);
 
 	/*
 	 * Syscall tracing and stepping should be turned off in the

@@ -3133,6 +3133,7 @@ do_sigaltstack (const stack_t __user *uss, stack_t __user *uoss, unsigned long s
 
 		current->sas_ss_sp = (unsigned long) ss_sp;
 		current->sas_ss_size = ss_size;
+		current->sas_ss_flags = ss_flags;
 	}
 
 	error = 0;

@@ -3163,9 +3164,14 @@ int restore_altstack(const stack_t __user *uss)
 int __save_altstack(stack_t __user *uss, unsigned long sp)
 {
 	struct task_struct *t = current;
-	return  __put_user((void __user *)t->sas_ss_sp, &uss->ss_sp) |
-		__put_user(sas_ss_flags(sp), &uss->ss_flags) |
+	int err = __put_user((void __user *)t->sas_ss_sp, &uss->ss_sp) |
+		__put_user(t->sas_ss_flags, &uss->ss_flags) |
 		__put_user(t->sas_ss_size, &uss->ss_size);
+	if (err)
+		return err;
+	if (t->sas_ss_flags & SS_AUTODISARM)
+		sas_ss_reset(t);
+	return 0;
 }
 
 #ifdef CONFIG_COMPAT