Commit ea3db3274d

Andrew Kelley <andrew@ziglang.org>
2022-07-27 05:05:54
link: avoid passing bad ptrs to pwritev
At least on Linux, the pwritev syscall checks the pointer and returns EFAULT before it checks if the length is nonzero. Perhaps this should be fixed in the standard library, however, these are still improvements since they make the kernel do less work within the syscall.
master
1 parent 0bc4726
Changed files (2)
src
link
C.zig
Dwarf.zig
src/link/C.zig
@@ -263,11 +263,13 @@ pub fn flushModule(self: *C, comp: *Compilation, prog_node: *std.Progress.Node)
     // Covers zig.h and err_typedef_item.
     try f.all_buffers.ensureUnusedCapacity(gpa, 2);
 
-    f.all_buffers.appendAssumeCapacity(.{
-        .iov_base = zig_h,
-        .iov_len = zig_h.len,
-    });
-    f.file_size += zig_h.len;
+    if (zig_h.len != 0) {
+        f.all_buffers.appendAssumeCapacity(.{
+            .iov_base = zig_h,
+            .iov_len = zig_h.len,
+        });
+        f.file_size += zig_h.len;
+    }
 
     const err_typedef_writer = f.err_typedef_buf.writer(gpa);
     const err_typedef_index = f.all_buffers.items.len;
@@ -301,11 +303,18 @@ pub fn flushModule(self: *C, comp: *Compilation, prog_node: *std.Progress.Node)
         try flushDecl(self, &f, decl_index);
     }
 
-    f.all_buffers.items[err_typedef_index] = .{
-        .iov_base = f.err_typedef_buf.items.ptr,
-        .iov_len = f.err_typedef_buf.items.len,
-    };
-    f.file_size += f.err_typedef_buf.items.len;
+    if (f.err_typedef_buf.items.len == 0) {
+        f.all_buffers.items[err_typedef_index] = .{
+            .iov_base = "",
+            .iov_len = 0,
+        };
+    } else {
+        f.all_buffers.items[err_typedef_index] = .{
+            .iov_base = f.err_typedef_buf.items.ptr,
+            .iov_len = f.err_typedef_buf.items.len,
+        };
+        f.file_size += f.err_typedef_buf.items.len;
+    }
 
     // Now the function bodies.
     try f.all_buffers.ensureUnusedCapacity(gpa, f.fn_count);
@@ -391,21 +400,25 @@ fn flushDecl(self: *C, f: *Flush, decl_index: Module.Decl.Index) FlushDeclError!
 
     if (decl_block.fwd_decl.items.len != 0) {
         const buf = decl_block.fwd_decl.items;
-        try f.all_buffers.append(gpa, .{
-            .iov_base = buf.ptr,
-            .iov_len = buf.len,
-        });
-        f.file_size += buf.len;
+        if (buf.len != 0) {
+            try f.all_buffers.append(gpa, .{
+                .iov_base = buf.ptr,
+                .iov_len = buf.len,
+            });
+            f.file_size += buf.len;
+        }
     }
     if (decl.getFunction() != null) {
         f.fn_count += 1;
     } else if (decl_block.code.items.len != 0) {
         const buf = decl_block.code.items;
-        try f.all_buffers.append(gpa, .{
-            .iov_base = buf.ptr,
-            .iov_len = buf.len,
-        });
-        f.file_size += buf.len;
+        if (buf.len != 0) {
+            try f.all_buffers.append(gpa, .{
+                .iov_base = buf.ptr,
+                .iov_len = buf.len,
+            });
+            f.file_size += buf.len;
+        }
     }
 }
 
@@ -421,19 +434,23 @@ pub fn flushEmitH(module: *Module) !void {
     defer all_buffers.deinit();
 
     var file_size: u64 = zig_h.len;
-    all_buffers.appendAssumeCapacity(.{
-        .iov_base = zig_h,
-        .iov_len = zig_h.len,
-    });
+    if (zig_h.len != 0) {
+        all_buffers.appendAssumeCapacity(.{
+            .iov_base = zig_h,
+            .iov_len = zig_h.len,
+        });
+    }
 
     for (emit_h.decl_table.keys()) |decl_index| {
         const decl_emit_h = emit_h.declPtr(decl_index);
         const buf = decl_emit_h.fwd_decl.items;
-        all_buffers.appendAssumeCapacity(.{
-            .iov_base = buf.ptr,
-            .iov_len = buf.len,
-        });
-        file_size += buf.len;
+        if (buf.len != 0) {
+            all_buffers.appendAssumeCapacity(.{
+                .iov_base = buf.ptr,
+                .iov_len = buf.len,
+            });
+            file_size += buf.len;
+        }
     }
 
     const directory = emit_h.loc.directory orelse module.comp.local_cache_directory;
src/link/Dwarf.zig
@@ -1752,7 +1752,7 @@ fn pwriteDbgLineNops(
         .iov_base = buf.ptr,
         .iov_len = buf.len,
     };
-    vec_index += 1;
+    if (buf.len > 0) vec_index += 1;
 
     {
         var padding_left = next_padding_size;
@@ -1861,7 +1861,7 @@ fn pwriteDbgInfoNops(
         .iov_base = buf.ptr,
         .iov_len = buf.len,
     };
-    vec_index += 1;
+    if (buf.len > 0) vec_index += 1;
 
     {
         var padding_left = next_padding_size;