Fix get_page_protections for openbsd since KERN_PROC_VMMAP works different

Author: jeremy-rifkin

src/platform/memory_mapping.cpp

@@ -113,8 +113,9 @@ namespace detail {
         return perms;
     }
     #elif defined(__FreeBSD__) || defined(__NetBSD__) || defined(__OpenBSD__)
+    #if !defined(__OpenBSD__)
     // Fetch VM mappings via sysctl with retry on ENOMEM.
-    // Freebsd uses a len * 4/3 heuristic in kinfo_getvmmap, we try to be more robust by retrying.
+    // FreeBSD uses a len * 4/3 heuristic in kinfo_getvmmap, we try to be more robust by retrying.
     // On all BSD I have investigated, sysctl returns ENOMEM when the buffer is too small (mappings grew between the
     // size query and the data fetch). When this happens, oldlenp gives the amount copied, not the amount needed, so we
     // must re-query the size from scratch.
@@ -128,7 +129,6 @@ namespace detail {
             auto original_len = len;
             // https://github.com/lattera/freebsd/blob/401a161083850a9a4ce916f37520c084cff1543b/lib/libutil/kinfo_getvmmap.c#L32C2-L32C20
             len = len * 4 / 3;
-            // OpenBSD requires the buffer size to be a multiple of sizeof(kinfo_vmentry)
             len -= len % sizeof(struct kinfo_vmentry);
             len = std::max(len, original_len);
             std::vector<char> buf(len);
@@ -142,6 +142,7 @@ namespace detail {
         }
         throw internal_error("sysctl vmmap failed after {} retries due to growing memory mappings", max_retries);
     }
+    #endif
     #if defined(__FreeBSD__)
     int get_page_protections(void* page) {
         int mib[4] = {CTL_KERN, KERN_PROC, KERN_PROC_VMMAP, getpid()};
@@ -191,20 +192,44 @@ namespace detail {
         );
     }
     #elif defined(__OpenBSD__)
+    // OpenBSD's KERN_PROC_VMMAP returns at most VMMAP_MAXLEN (64KB) per call and rejects larger buffers with EINVAL.
+    // The API is paginated, setting kve_start in the first buffer entry tells the kernel where to resume.
     int get_page_protections(void* page) {
         int mib[3] = {CTL_KERN, KERN_PROC_VMMAP, getpid()};
-        auto buf = sysctl_vmmap(mib, 3);
+        size_t buf_size = 0;
+        if(sysctl(mib, 3, nullptr, &buf_size, nullptr, 0) != 0) {
+            throw internal_error("sysctl vmmap size query failed: {}", strerror(errno));
+        }
+        buf_size -= buf_size % sizeof(struct kinfo_vmentry);
         auto addr = reinterpret_cast<uintptr_t>(page);
-        auto count = buf.size() / sizeof(struct kinfo_vmentry);
-        auto* entries = reinterpret_cast<struct kinfo_vmentry*>(buf.data());
-        for(size_t i = 0; i < count; i++) {
-            if(addr >= entries[i].kve_start && addr < entries[i].kve_end) {
-                int perms = 0;
-                if(entries[i].kve_protection & KVE_PROT_READ)  perms |= PROT_READ;
-                if(entries[i].kve_protection & KVE_PROT_WRITE) perms |= PROT_WRITE;
-                if(entries[i].kve_protection & KVE_PROT_EXEC)  perms |= PROT_EXEC;
-                return perms;
+        std::vector<char> buf(buf_size);
+        unsigned long next_start = 0;
+        while(true) {
+            reinterpret_cast<struct kinfo_vmentry*>(buf.data())->kve_start = next_start;
+            size_t len = buf_size;
+            if(sysctl(mib, 3, buf.data(), &len, nullptr, 0) != 0) {
+                throw internal_error("sysctl vmmap failed: {}", strerror(errno));
+            }
+            if(len == 0) {
+                break;
+            }
+            auto count = len / sizeof(struct kinfo_vmentry);
+            auto* entries = reinterpret_cast<struct kinfo_vmentry*>(buf.data());
+            for(size_t i = 0; i < count; i++) {
+                if(addr >= entries[i].kve_start && addr < entries[i].kve_end) {
+                    int perms = 0;
+                    if(entries[i].kve_protection & KVE_PROT_READ)  perms |= PROT_READ;
+                    if(entries[i].kve_protection & KVE_PROT_WRITE) perms |= PROT_WRITE;
+                    if(entries[i].kve_protection & KVE_PROT_EXEC)  perms |= PROT_EXEC;
+                    return perms;
+                }
+            }
+            if(len < buf_size) {
+                break;
             }
+            // basic sanity check for forward progress
+            VERIFY(next_start == 0 || entries[count - 1].kve_end > next_start);
+            next_start = entries[count - 1].kve_end;
         }
         throw internal_error(
             "Failed to find mapping for {>16:0h} via sysctl KERN_PROC_VMMAP",