Unnecessary process.args/env coping when spawning a child?

The following TODO arouses my interest. Why we need to deep copy args and env to heap? The parent can not access or even modify the copy of them in the child once uv_spawn succeed, in case it failed, libuv does not(and has no reason) to touch them.

Code to prepare uv_process_options_t before call uv_spawn() in the implementation of child_process.spawn :

node/src/process_wrap.cc

Lines 178 to 204 in b3f5a41

    
           // TODO(bnoordhuis) is this possible to do without mallocing ? 
        
           // options.file 
        
           Local<Value> file_v = 
        
               js_options->Get(context, env->file_string()).ToLocalChecked(); 
        
           CHECK(file_v->IsString()); 
        
           node::Utf8Value file(env->isolate(), file_v); 
        
           options.file = *file; 
        
           // options.args 
        
           Local<Value> argv_v = 
        
               js_options->Get(context, env->args_string()).ToLocalChecked(); 
        
           if (!argv_v.IsEmpty() && argv_v->IsArray()) { 
        
             Local<Array> js_argv = argv_v.As<Array>(); 
        
             int argc = js_argv->Length(); 
        
             CHECK_GT(argc + 1, 0);  // Check for overflow. 
        
             // Heap allocate to detect errors. +1 is for nullptr. 
        
             options.args = new char*[argc + 1]; 
        
             for (int i = 0; i < argc; i++) { 
        
               node::Utf8Value arg(env->isolate(), 
        
                                   js_argv->Get(context, i).ToLocalChecked()); 
        
               options.args[i] = strdup(*arg); 
        
               CHECK_NOT_NULL(options.args[i]); 
        
             } 
        
             options.args[argc] = nullptr; 
        
           }

Code to prepare uv_process_options_t before call uv_spawn() in the implementation of child_process.spawnSync :

node/src/spawn_sync.cc

Lines 755 to 765 in b3f5a41

    
           Local<Value> js_file = 
        
               js_options->Get(context, env()->file_string()).ToLocalChecked(); 
        
           if (!CopyJsString(js_file, &file_buffer_).To(&r)) return Nothing<int>(); 
        
           if (r < 0) return Just(r); 
        
           uv_process_options_.file = file_buffer_; 
        
           Local<Value> js_args = 
        
               js_options->Get(context, env()->args_string()).ToLocalChecked(); 
        
           if (!CopyJsStringArray(js_args, &args_buffer_).To(&r)) return Nothing<int>(); 
        
           if (r < 0) return Just(r); 
        
           uv_process_options_.args = reinterpret_cast<char**>(args_buffer_);

Libuv's uv_spawn() (and the underlying execve() system call) expects the arguments and environment as a char*[] with a nullptr sentinel, that's why.

The code has changed over the years but the comment persisted. Maybe it should just be removed.

Something I noticed in the surrounding code:

CHECK_GT(argc + 1, 0);  // Check for overflow.

I think that's trying to check that argc != INT_MAX but the way it's written is UB.

@bnoordhuis

@bnoordhuis Thanks for the information. The code makes sense to me now :)

src: fix UB in overflow checks

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Unnecessary process.args/env coping when spawning a child? #45868

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	// TODO(bnoordhuis) is this possible to do without mallocing ?

	// options.file
	Local<Value> file_v =
	js_options->Get(context, env->file_string()).ToLocalChecked();
	CHECK(file_v->IsString());
	node::Utf8Value file(env->isolate(), file_v);
	options.file = *file;

	// options.args
	Local<Value> argv_v =
	js_options->Get(context, env->args_string()).ToLocalChecked();
	if (!argv_v.IsEmpty() && argv_v->IsArray()) {
	Local<Array> js_argv = argv_v.As<Array>();
	int argc = js_argv->Length();
	CHECK_GT(argc + 1, 0); // Check for overflow.

	// Heap allocate to detect errors. +1 is for nullptr.
	options.args = new char*[argc + 1];
	for (int i = 0; i < argc; i++) {
	node::Utf8Value arg(env->isolate(),
	js_argv->Get(context, i).ToLocalChecked());
	options.args[i] = strdup(*arg);
	CHECK_NOT_NULL(options.args[i]);
	}
	options.args[argc] = nullptr;
	}

	Local<Value> js_file =
	js_options->Get(context, env()->file_string()).ToLocalChecked();
	if (!CopyJsString(js_file, &file_buffer_).To(&r)) return Nothing<int>();
	if (r < 0) return Just(r);
	uv_process_options_.file = file_buffer_;

	Local<Value> js_args =
	js_options->Get(context, env()->args_string()).ToLocalChecked();
	if (!CopyJsStringArray(js_args, &args_buffer_).To(&r)) return Nothing<int>();
	if (r < 0) return Just(r);
	uv_process_options_.args = reinterpret_cast<char**>(args_buffer_);

Unnecessary process.args/env coping when spawning a child? #45868

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bnoordhuis commented on Dec 16, 2022

ywave620 commented on Dec 16, 2022

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