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Fix PNG loader not resetting stream position on error right after reading signature #387

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Merged
merged 3 commits into from
Jan 25, 2025
Merged

Fix PNG loader not resetting stream position on error right after reading signature #387

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f44d2f0
Core (LV::Video): Fix PNG loader not resetting stream position on err…
kaixiong Jan 24, 2025
2cfeb2f
Core (LV::Video): Fix and clarify the various seeks in the PNG loader.
kaixiong Jan 24, 2025
2ef768f
Core (LV::Video): Use 'delete[] ptr' instead of 'delete []ptr'.
kaixiong Jan 24, 2025
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28 changes: 23 additions & 5 deletions libvisual/libvisual/private/lv_video_png.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -58,13 +58,24 @@ namespace LV {

VideoPtr bitmap_load_png (std::istream& input)
{
auto saved_stream_pos = input.tellg ();
auto start_stream_pos = input.tellg ();

// Check PNG signature.

png_byte signature[8];
input.read (reinterpret_cast<char*> (signature), sizeof (signature));
if (!input.read (reinterpret_cast<char*> (signature), sizeof (signature))) {
input.clear ();
input.seekg (start_stream_pos);
return nullptr;
}

bool is_png = !png_sig_cmp (signature, 0, sizeof (signature));

// Clean up test by rewinding to the beginning, like we have read nothing.
if (!input.seekg (start_stream_pos)) {
return nullptr;
}

if (!is_png) {
return nullptr;
}
Expand All @@ -86,15 +97,22 @@ namespace LV {
return nullptr;
}

// Read PNG image data

// Skip to the first chunk, which comes right after the signature.
input.seekg (start_stream_pos + std::streampos {sizeof (signature)});
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@hartwork hartwork Jan 24, 2025

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Suggested change
input.seekg (start_stream_pos + std::streampos {sizeof (signature)});
if (!input.seekg (start_stream_pos + std::streampos {sizeof (signature)})) {
png_destroy_read_struct (&png_ptr, &info_ptr, &end_info);
return nullptr;
}

?

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@kaixiong kaixiong Jan 25, 2025

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The subsequent read() calls (called via libpng hooks) will continue to keep the fail flag and trigger the cleanup in the setjmp() block, which also resets the stream position.

Specifically, read() first checks if good() is false (this happens if the seek fails), sets the fail flag again before returning.

One thing about the iostream (with exceptions off) is that once an operation fails, it basically stops trying to do anything. The remaining operations become effectively no-ops. So, one can safely issue a series of operations (e.g. read) and only check the status at the end.

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@hartwork hartwork Jan 25, 2025

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@kaixiong I think you're saying that you consider status quo good enough. That's alright with me, I will approve now…

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@kaixiong kaixiong Jan 25, 2025

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@kaixiong I think you're saying that you consider status quo good enough. That's alright with me, I will approve now…

@hartwork Thank you.

In case I wasn't clear, there's a subtle difference between your suggested change and the original.

Namely that, with the change, when that seek() does fails, in particular when there's no more data to read (e.g. immediate end-of-file after signature), the handler fails to reset the stream position (only freeing the read struct).

The status quo on the other hand, will go through the entire cleanup process triggered on the first subsequent read() that is guaranteed to fail.


uint8_t* pixels = nullptr;
uint8_t** pixel_row_ptrs = nullptr;

if (setjmp (png_jmpbuf (png_ptr))) {
input.seekg (saved_stream_pos);
// Some error happened during reading. Rewind to the beginning, like we have read nothing.
input.clear ();
input.seekg (start_stream_pos);

png_destroy_read_struct (&png_ptr, &info_ptr, &end_info);

delete []pixel_row_ptrs;
delete[] pixel_row_ptrs;
visual_mem_free (pixels);

return nullptr;
Expand Down Expand Up @@ -171,7 +189,7 @@ namespace LV {

png_destroy_read_struct (&png_ptr, &info_ptr, &end_info);

delete []pixel_row_ptrs;
delete[] pixel_row_ptrs;

return Video::wrap (pixels, true, width, height, depth);
}
Expand Down