altstack: stack alignment and accounting tweaks

* add altstack_remn, returns amount of stack remaining
* increase mapping by 1 page to handle abutment case
* capture rsp earlier
* align stack to 16 bytes

Signed-off-by: Dan Good <dan@dancancode.com>

ccan/altstack/altstack.c

@@ -8,6 +8,7 @@
 #include <signal.h>
 #include <stdio.h>
 #include <string.h>
+#include <unistd.h>
 #include <sys/mman.h>
 
 static __thread char ebuf[ALTSTACK_ERR_MAXLEN];
@@ -37,6 +38,11 @@ static void segvjmp(int signum)
 }
 
 static __thread void *rsp_save_[2];
+static __thread rlim_t max_;
+
+rlim_t altstack_max(void) {
+	return max_;
+}
 
 static ptrdiff_t rsp_save(unsigned i) {
 	assert(i < 2);
@@ -57,6 +63,7 @@ static __thread void *arg_, *out_;
 
 int altstack(rlim_t max, void *(*fn)(void *), void *arg, void **out)
 {
+	long pgsz = sysconf(_SC_PAGESIZE);
 	int ret = -1, undo = 0;
 	char *m;
 	struct rlimit rl_save;
@@ -69,11 +76,16 @@ int altstack(rlim_t max, void *(*fn)(void *), void *arg, void **out)
 	fn_  = fn;
 	arg_ = arg;
 	out_ = 0;
+	max_ = max;
 	ebuf[elen = 0] = '\0';
 	if (out) *out = 0;
 
+	// if the first page below the mapping is in use, we get max-pgsz usable bytes
+	// add pgsz to max to guarantee at least max usable bytes
+	max += pgsz;
+
 	ok(getrlimit(RLIMIT_STACK, &rl_save), 1);
-	ok(setrlimit(RLIMIT_STACK, &(struct rlimit) { max, rl_save.rlim_max }), 1);
+	ok(setrlimit(RLIMIT_STACK, &(struct rlimit) { max_, rl_save.rlim_max }), 1);
 	undo++;
 
 	ok(m = mmap(0, max, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS|MAP_GROWSDOWN|MAP_NORESERVE, -1, 0), 1);
@@ -91,8 +103,12 @@ int altstack(rlim_t max, void *(*fn)(void *), void *arg, void **out)
 		ok(sigaction(SIGSEGV, &sa, &sa_save), 1);
 		undo++;
 
-		asm volatile ("movq %%rsp, %%r10\nmov %0, %%rsp\npush %%r10" : : "g" (m + max) : "r10");
-		rsp_save(0);
+		asm volatile (
+			"mov %%rsp, %%r10\n\t"
+			"mov %1, %%rsp\n\t"
+			"sub $8, %%rsp\n\t"
+			"push %%r10"
+			: "=r" (rsp_save_[0]) : "0" (m + max) : "r10");
 		out_ = fn_(arg_);
 		asm volatile ("pop %rsp");
 		ret = 0;

ccan/altstack/altstack.h

@@ -103,6 +103,20 @@ char *altstack_geterr(void);
  */
 ptrdiff_t altstack_used(void);
 
+/**
+ * altstack_max - return usable stack size
+ *
+ * Returns: max value from altstack() call
+ */
+rlim_t altstack_max(void);
+
+/**
+ * altstack_remn - return amount of stack remaining
+ *
+ * Returns: altstack_max() minus altstack_used()
+ */
+#define altstack_remn() (altstack_max() - altstack_used())
+
 /**
  * altstack_rsp_save - set initial rsp value
  *

ccan/altstack/test/run.c

@@ -4,6 +4,7 @@
 #include <setjmp.h>
 #include <signal.h>
 #include <string.h>
+#include <unistd.h>
 #include <sys/mman.h>
 #include <ccan/tap/tap.h>
 #include <ccan/altstack/altstack.h>
@@ -20,13 +21,13 @@ enum {
 };
 int fail, call1, call2;
 char *m_;
-rlim_t max_;
+rlim_t msz_;
 #define e(x) (900+(x))
 #define seterr(x) (errno = e(x))
 #define setcall(x) ((call1 |= !errno ? (x) : 0), (call2 |= errno || out_ ? (x) : 0))
 #define getrlimit(...)		(fail&getrlimit_	? (seterr(getrlimit_),		-1) : (setcall(getrlimit_),	getrlimit(__VA_ARGS__)))
 #define mmap(...)		(fail&mmap_		? (seterr(mmap_),	(void *)-1) : (setcall(mmap_),		mmap(__VA_ARGS__)))
-#define munmap(a, b)		(fail&munmap_		? (seterr(munmap_),		-1) : (setcall(munmap_),	munmap(m_=(a), max_=(b))))
+#define munmap(a, b)		(fail&munmap_		? (seterr(munmap_),		-1) : (setcall(munmap_),	munmap(m_=(a), msz_=(b))))
 #define setrlimit(...)		(fail&setrlimit_	? (seterr(setrlimit_),		-1) : (setcall(setrlimit_),	setrlimit(__VA_ARGS__)))
 #define sigaltstack(...)	(fail&sigaltstack_	? (seterr(sigaltstack_),	-1) : (setcall(sigaltstack_),	sigaltstack(__VA_ARGS__)))
 #define sigaction(...)		(fail&sigaction_	? (seterr(sigaction_),		-1) : (setcall(sigaction_),	sigaction(__VA_ARGS__)))
@@ -58,7 +59,9 @@ static void *wrap(void *i)
 
 int main(void)
 {
-	plan_tests(16);
+	long pgsz = sysconf(_SC_PAGESIZE);
+
+	plan_tests(17);
 
 #define chkfail(x, y, z, c1, c2) (call1 = 0, call2 = 0, errno = 0, ok1((fail = x) && (y) && errno == (z) && call1 == (c1) && call2 == (c2)));
 #define   chkok(   y, z, c1, c2) (call1 = 0, call2 = 0, errno = 0, fail = 0,     ok1((y) && errno == (z) && call1 == (c1) && call2 == (c2)));
@@ -86,7 +89,7 @@ int main(void)
 	chkfail(munmap_,	altstack(8*MiB, wrap, 0, 0) ==  1, e(munmap_),
 		getrlimit_|setrlimit_|mmap_|sigaltstack_|sigaction_,
 		setrlimit_|sigaltstack_|sigaction_);
-	if (fail = 0, munmap(m_, max_) == -1)
+	if (fail = 0, munmap(m_, msz_) == -1)
 		err(1, "munmap");
 
 	chkok(			altstack(1*MiB, wrap, (void *) 1000000, 0) == -1, EOVERFLOW,
@@ -102,10 +105,12 @@ int main(void)
 	chkfail(munmap_,	altstack(1*MiB, wrap, (void *) 1000000, 0) == -1, EOVERFLOW,
 		getrlimit_|setrlimit_|mmap_|sigaltstack_|sigaction_,
 		setrlimit_|sigaltstack_|sigaction_);
-	if (fail = 0, munmap(m_, max_) == -1)
+	if (fail = 0, munmap(m_, msz_) == -1)
 		err(1, "munmap");
 
-	ok1(used > 1*MiB-1*KiB && used < 1*MiB);
+	ok1(altstack_max() == 1*MiB);
+	diag("used: %lu", used);
+	ok1(used >= 1*MiB - pgsz && used <= 1*MiB + pgsz);
 
 	char *p;
 	for(p = altstack_geterr(); *p; p++)
@@ -128,7 +133,8 @@ int main(void)
 		getrlimit_|setrlimit_|mmap_|sigaltstack_|sigaction_,
 		setrlimit_|munmap_|sigaltstack_|sigaction_);
 
-	ok1(used > 8*MiB-8*KiB && used < 8*MiB);
+	diag("used: %lu", used);
+	ok1(used >= 8*MiB - pgsz && used <= 8*MiB + pgsz);
 
 	used = 0;
 	chkok(			altstack(8*MiB, wrap, (void *) 100000, 0) == 0, 0,
@@ -138,7 +144,8 @@ int main(void)
 	used = 1;
 	altstack_rsp_save();
 	dn(0);
-	ok1(used == 32);
+	diag("used: %lu", used);
+	ok1(used == 32 || used == 40);
 
 	return exit_status();
 }