altstack: stack alignment and accounting tweaks

[ccan] / ccan / altstack / altstack.c

diff --git a/ccan/altstack/altstack.c b/ccan/altstack/altstack.c
index 6faf38f5d31d8870e82db4d96ea2e657944f59bd..67f457bce3b90dd6cdf406b33e0ec4ffd8de0834 100644 (file)
--- a/ccan/altstack/altstack.c
+++ b/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;