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X11: Fix undefined behavior in glfwSetWindowIcon

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The conversion of window icon image data involves unsigned char color
values being promoted to int and then shifted to the left by 24.  For
32-bit ints this is just far enough to trigger undefined behavior.

It worked by accident because of how current compilers translate this
piece of code.

This was caught by @slimsag while working on [Zig bindings for GLFW][1],
and diagnosed together with @Andoryuuta, as described [in an
article][2].  Zig has UBSan enabled by default, which caught this
undefined behavior.

[1]: https://github.com/hexops/mach-glfw
[2]: https://devlog.hexops.com/2021/perfecting-glfw-for-zig-and-finding-undefined-behavior#finding-lurking-undefined-behavior-in-6-year-old-glfw-code

Thanks to Maato, martinhath, dcousens, drfuchs and Validark for helping
to refine the solution.

This commit message was rewritten by @elmindreda to hopefully reflect
the conclusions of the pull request thread.

Related to hexops/mach#20
Closes #1986
This commit is contained in:
Stephen Gutekanst 2021-10-31 11:22:40 -07:00 committed by Camilla Löwy
parent 37fc28bff6
commit 9cd4d2fa20
1 changed files with 13 additions and 6 deletions
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19
src/x11_window.c
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@ -2105,8 +2105,8 @@ void _glfwSetWindowIconX11(_GLFWwindow* window, int count, const GLFWimage* imag
for (int i = 0; i < count; i++)
longCount += 2 + images[i].width * images[i].height;
long* icon = _glfw_calloc(longCount, sizeof(long));
long* target = icon;
unsigned long* icon = _glfw_calloc(longCount, sizeof(unsigned long));
unsigned long* target = icon;
for (int i = 0; i < count; i++)
{
@ -2115,13 +2115,20 @@ void _glfwSetWindowIconX11(_GLFWwindow* window, int count, const GLFWimage* imag
for (int j = 0; j < images[i].width * images[i].height; j++)
{
*target++ = (images[i].pixels[j * 4 + 0] << 16) |
(images[i].pixels[j * 4 + 1] << 8) |
(images[i].pixels[j * 4 + 2] << 0) |
(images[i].pixels[j * 4 + 3] << 24);
*target++ = (((unsigned long)images[i].pixels[j * 4 + 0]) << 16) |
(((unsigned long)images[i].pixels[j * 4 + 1]) << 8) |
(((unsigned long)images[i].pixels[j * 4 + 2]) << 0) |
(((unsigned long)images[i].pixels[j * 4 + 3]) << 24);
}
}
// Important: Despite XChangeProperty docs indicating that `icon` (unsigned char*) would be
// in the format of the icon image, e.g. 32-bit below, the function actually casts the ptr
// (unsigned char*) internally to (long*) and then if long is defined as 64-bits, as on IL64
// platforms, extracts only 32 bits from the long leaving the other 32 unused. That is, on a
// 64-bit platform XChangeProperty expects 64-bit integers representing 32-bit pixels.
//
// See https://github.com/glfw/glfw/pull/1986#issuecomment-962445299
XChangeProperty(_glfw.x11.display, window->x11.handle,
_glfw.x11.NET_WM_ICON,
XA_CARDINAL, 32,