webgl: fix glGetUniform*v ignoring program param (#26845)

vittorioromeo · web-flow · commit ca13dbce0fa9 · 2026-05-08T11:29:58.000+03:00
Fixes #26844. PR created with AI assistance. `$emscriptenWebGLGetUniform` resolved its uniform location via `$webglGetUniformLocation`, which uses `GLctx.currentProgram`. Per the GLES spec, `glGetUniform*v` takes the program explicitly and does not require `glUseProgram`. Calling it without a current program returned undefined and `GLctx.getUniform` threw a `TypeError`. Additionally, under `-sGL_ASSERTIONS`, the location validation ran before the integer-`GLuint` -> `WebGLProgram` swap, dereferencing a property of a `Number` and crashing. Fix both: run assertions in the correct order around the swap and prepare call, and resolve the location against the explicit program inline. Adds a parameterized regression test.
diff --git a/src/lib/libhtml5_webgl.js b/src/lib/libhtml5_webgl.js
@@ -514,14 +514,14 @@ var LibraryHtml5WebGL = {
     writeGLArray(GLctx.getVertexAttrib(index, param), dst, dstLength, dstType),
 
   emscripten_webgl_get_uniform_d__proxy: 'sync_on_current_webgl_context_thread',
-  emscripten_webgl_get_uniform_d__deps: ['$webglGetUniformLocation'],
+  emscripten_webgl_get_uniform_d__deps: ['$webglGetProgramUniformLocation'],
   emscripten_webgl_get_uniform_d: (program, location) =>
-    GLctx.getUniform(GL.programs[program], webglGetUniformLocation(location)),
+    GLctx.getUniform(GL.programs[program], webglGetProgramUniformLocation(GL.programs[program], location)),
 
   emscripten_webgl_get_uniform_v__proxy: 'sync_on_current_webgl_context_thread',
-  emscripten_webgl_get_uniform_v__deps: ['$writeGLArray', '$webglGetUniformLocation'],
+  emscripten_webgl_get_uniform_v__deps: ['$writeGLArray', '$webglGetProgramUniformLocation'],
   emscripten_webgl_get_uniform_v: (program, location, dst, dstLength, dstType) =>
-    writeGLArray(GLctx.getUniform(GL.programs[program], webglGetUniformLocation(location)), dst, dstLength, dstType),
+    writeGLArray(GLctx.getUniform(GL.programs[program], webglGetProgramUniformLocation(GL.programs[program], location)), dst, dstLength, dstType),
 
   emscripten_webgl_get_parameter_v__proxy: 'sync_on_current_webgl_context_thread',
   emscripten_webgl_get_parameter_v__deps: ['$writeGLArray'],
diff --git a/src/lib/libwebgl.js b/src/lib/libwebgl.js
@@ -2121,7 +2121,7 @@ for (/**@suppress{duplicate}*/var i = 0; i <= {{{ GL_POOL_TEMP_BUFFERS_SIZE }}};
   // This function intentionally assigns `HEAP32[x] = someBoolean;` Don't let
   // Closure mind about that.
   $emscriptenWebGLGetUniform__docs: '/** @suppress{checkTypes} */',
-  $emscriptenWebGLGetUniform__deps: ['$webglGetUniformLocation', '$webglPrepareUniformLocationsBeforeFirstUse'],
+  $emscriptenWebGLGetUniform__deps: ['$webglGetProgramUniformLocation', '$webglPrepareUniformLocationsBeforeFirstUse'],
   $emscriptenWebGLGetUniform: (program, location, params, type) => {
     if (!params) {
       // GLES2 specification does not specify how to behave if params is a null
@@ -2139,7 +2139,7 @@ for (/**@suppress{duplicate}*/var i = 0; i <= {{{ GL_POOL_TEMP_BUFFERS_SIZE }}};
 #endif
     program = GL.programs[program];
     webglPrepareUniformLocationsBeforeFirstUse(program);
-    var data = GLctx.getUniform(program, webglGetUniformLocation(location));
+    var data = GLctx.getUniform(program, webglGetProgramUniformLocation(program, location));
     if (typeof data == 'number' || typeof data == 'boolean') {
       switch (type) {
         case {{{ cDefs.EM_FUNC_SIG_PARAM_I }}}: {{{ makeSetValue('params', '0', 'data', 'i32') }}}; break;
@@ -2173,34 +2173,32 @@ for (/**@suppress{duplicate}*/var i = 0; i <= {{{ GL_POOL_TEMP_BUFFERS_SIZE }}};
 
   // Returns the WebGLUniformLocation object corresponding to the location index
   // integer on the currently active shader in this GL context.
-  $webglGetUniformLocation__deps: ['$webglPrepareUniformLocationsBeforeFirstUse'],
-  $webglGetUniformLocation: (location) => {
-    var p = GLctx.currentProgram;
-
+  $webglGetProgramUniformLocation__deps: ['$webglPrepareUniformLocationsBeforeFirstUse'],
+  $webglGetProgramUniformLocation: (program, location) => {
 #if !GL_TRACK_ERRORS && ASSERTIONS
     // In -sGL_TRACK_ERRORS=0 build mode do not allow calling glUniform*()
     // without an active GL program.
-    assert(p, 'Attempted to call glUniform*() without an active GL program set! (build with -sGL_TRACK_ERRORS for standards-conformant behavior)');
+    assert(program, 'When building with !GL_TRACK_ERRORS, program cannot be null, in a call to webglGetProgramUniformLocation()');
 #endif
 
 #if GL_TRACK_ERRORS
-    if (p) {
+    if (program) {
 #endif
 #if GL_EXPLICIT_UNIFORM_LOCATION
       // Ensure `uniformLocsById`/`uniformArrayNamesById` are populated. Without
       // this, calling `glUniform*()` on a freshly linked program before any
       // `glGetUniformLocation()` silently no-ops: `glLinkProgram` resets
       // `uniformLocsById` to 0 and only `$webglPrepareUniformLocationsBeforeFirstUse`
       // refills it. The call below is idempotent (guards on `!uniformLocsById`).
-      webglPrepareUniformLocationsBeforeFirstUse(p);
+      webglPrepareUniformLocationsBeforeFirstUse(program);
 #endif
-      var webglLoc = p.uniformLocsById[location];
-      // p.uniformLocsById[location] stores either an integer, or a
+      var webglLoc = program.uniformLocsById[location];
+      // program.uniformLocsById[location] stores either an integer, or a
       // WebGLUniformLocation.
       // If an integer, we have not yet bound the location, so do it now. The
       // integer value specifies the array index we should bind to.
       if (typeof webglLoc == 'number') {
-        p.uniformLocsById[location] = webglLoc = GLctx.getUniformLocation(p, p.uniformArrayNamesById[location] + (webglLoc > 0 ? `[${webglLoc}]` : ''));
+        program.uniformLocsById[location] = webglLoc = GLctx.getUniformLocation(program, program.uniformArrayNamesById[location] + (webglLoc > 0 ? `[${webglLoc}]` : ''));
       }
       // Else an already cached WebGLUniformLocation, return it.
       return webglLoc;
@@ -2211,6 +2209,17 @@ for (/**@suppress{duplicate}*/var i = 0; i <= {{{ GL_POOL_TEMP_BUFFERS_SIZE }}};
 #endif
   },
 
+  $webglGetUniformLocation__deps: ['$webglGetProgramUniformLocation'],
+  $webglGetUniformLocation: (location) => {
+#if !GL_TRACK_ERRORS && ASSERTIONS
+    // In -sGL_TRACK_ERRORS=0 build mode do not allow calling glUniform*()
+    // without an active GL program.
+    assert(GLctx.currentProgram, 'Attempted to call glUniform*()/webglGetUniformLocation() without an active GL program set! (build with -sGL_TRACK_ERRORS for standards-conformant behavior)');
+#endif
+
+    return webglGetProgramUniformLocation(GLctx.currentProgram, location);
+  },
+
   $webglPrepareUniformLocationsBeforeFirstUse__deps: ['$webglGetLeftBracePos'],
   $webglPrepareUniformLocationsBeforeFirstUse: (program) => {
     var uniformLocsById = program.uniformLocsById, // Maps GLuint -> WebGLUniformLocation
diff --git a/test/browser/webgl_get_uniform_no_active_program.c b/test/browser/webgl_get_uniform_no_active_program.c
@@ -0,0 +1,83 @@
+// Copyright 2026 The Emscripten Authors.  All rights reserved.
+// Emscripten is available under two separate licenses, the MIT license and the
+// University of Illinois/NCSA Open Source License.  Both these licenses can be
+// found in the LICENSE file.
+
+// Regression test for two bugs in $emscriptenWebGLGetUniform (libwebgl.js):
+//
+//   1. glGetUniform*v resolved its uniform location against
+//      `GLctx.currentProgram` instead of the explicit `program` parameter, so
+//      calling glGetUniform*v without a prior glUseProgram(program) threw
+//      "parameter 2 is not of type 'WebGLUniformLocation'" from
+//      GLctx.getUniform. Per the GLES spec, glGetUniform*v takes the program
+//      explicitly and does not require it to be bound.
+//
+//   2. Under -sGL_ASSERTIONS, the location validation ran before the integer
+//      ID -> WebGLProgram lookup, dereferencing a property of a number and
+//      throwing "Cannot read properties of undefined (reading '<location>')".
+
+#include <assert.h>
+#include <stdio.h>
+#include <GLES3/gl3.h>
+#include <emscripten/html5.h>
+
+int main() {
+  EmscriptenWebGLContextAttributes attr;
+  emscripten_webgl_init_context_attributes(&attr);
+  attr.majorVersion = 2;
+  EMSCRIPTEN_WEBGL_CONTEXT_HANDLE ctx = emscripten_webgl_create_context("#canvas", &attr);
+  assert(ctx);
+  emscripten_webgl_make_context_current(ctx);
+
+  const char* vs = "#version 300 es\nvoid main(){gl_Position=vec4(0);}";
+  const char* fs = "#version 300 es\n"
+                   "precision mediump float;\n"
+                   "uniform float u;\n"
+                   "out vec4 o;\n"
+                   "void main(){ o = vec4(u); }";
+
+  GLuint v = glCreateShader(GL_VERTEX_SHADER);
+  glShaderSource(v, 1, &vs, NULL);
+  glCompileShader(v);
+
+  GLuint f = glCreateShader(GL_FRAGMENT_SHADER);
+  glShaderSource(f, 1, &fs, NULL);
+  glCompileShader(f);
+
+  GLuint p = glCreateProgram();
+  glAttachShader(p, v);
+  glAttachShader(p, f);
+  glLinkProgram(p);
+
+  GLint linked = 0;
+  glGetProgramiv(p, GL_LINK_STATUS, &linked);
+  assert(linked && "Program link failed");
+
+  GLint loc = glGetUniformLocation(p, "u");
+  assert(loc >= 0);
+
+  glUseProgram(p);
+  glUniform1f(loc, 1.5f);
+  assert(glGetError() == GL_NO_ERROR);
+
+  // Case A: program currently bound. Without the fix, this crashes under
+  // -sGL_ASSERTIONS=1 because validateGLObjectID(program.uniformLocsById, ...)
+  // runs before the integer -> WebGLProgram swap.
+  float outA = -1.f;
+  glGetUniformfv(p, loc, &outA);
+  assert(glGetError() == GL_NO_ERROR);
+  assert(outA == 1.5f);
+
+  // Case B: no program currently bound. Without the fix, glGetUniform*v
+  // resolves `location` against GLctx.currentProgram (null), the inner GL
+  // call receives `undefined` for the WebGLUniformLocation, and WebGL throws
+  // a TypeError.
+  glUseProgram(0);
+  float outB = -1.f;
+  glGetUniformfv(p, loc, &outB);
+  assert(glGetError() == GL_NO_ERROR);
+  assert(outB == 1.5f);
+
+  printf("Test passed!\n");
+  return 0;
+}
diff --git a/test/test_browser.py b/test/test_browser.py
@@ -1243,6 +1243,14 @@ def test_webgl_explicit_uniform_location(self):
   def test_webgl_uniform_before_get_location(self, args):
     self.btest_exit('webgl_uniform_before_get_location.c', cflags=args + ['-sGL_EXPLICIT_UNIFORM_LOCATION', '-sMIN_WEBGL_VERSION=2'])
 
+  @parameterized({
+    '': ([],),
+    'assertions': (['-sGL_ASSERTIONS'],),
+  })
+  @requires_webgl2
+  def test_webgl_get_uniform_no_active_program(self, args):
+    self.btest_exit('webgl_get_uniform_no_active_program.c', cflags=args + ['-sMIN_WEBGL_VERSION=2'])
+
   @requires_graphics_hardware
   def test_webgl_sampler_layout_binding(self):
     self.btest_exit('webgl_sampler_layout_binding.c', cflags=['-sGL_EXPLICIT_UNIFORM_BINDING'])
