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fix: Use wrapping arithmetic in PNG to avoid overflow 

Use +% for wrapping addition to prevent debug panic on overflow.
Cast through u32 for multiplication to avoid u8 overflow.

Refs: RFC-0015
This commit is contained in:
Markus Maiwald 2026-02-03 17:50:08 +01:00
parent 4bd581dd71
commit ef0b7b61f6
1 changed files with 8 additions and 3 deletions
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11
l0-transport/png.zig
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@ -70,15 +70,20 @@ pub const PngState = struct {
const size_dist_val = entropy[0] % 4; const size_dist_val = entropy[0] % 4;
const timing_dist_val = entropy[1] % 3; const timing_dist_val = entropy[1] % 3;
// Use wrapping arithmetic to avoid overflow panics in debug mode
const size_mean_val = @as(u16, 1200) +% @as(u16, @as(u32, entropy[2]) * 2);
const size_stddev_val = @as(u16, 100) +% @as(u16, entropy[3]);
const epoch_count = @as(u32, 100) +% (@as(u32, entropy[7]) * 4);
return EpochProfile{ return EpochProfile{
.size_distribution = @enumFromInt(@as(u32, size_dist_val)), .size_distribution = @enumFromInt(@as(u32, size_dist_val)),
.size_mean = @as(u16, 1200) + (@as(u16, entropy[2]) * 2), // 1200-1710 bytes .size_mean = size_mean_val,
.size_stddev = 100 + entropy[3], // 100-355 bytes .size_stddev = size_stddev_val,
.timing_distribution = @enumFromInt(@as(u32, timing_dist_val)), .timing_distribution = @enumFromInt(@as(u32, timing_dist_val)),
.timing_lambda = 0.001 + (@as(f64, @floatFromInt(entropy[4])) / 255.0) * 0.019, .timing_lambda = 0.001 + (@as(f64, @floatFromInt(entropy[4])) / 255.0) * 0.019,
.dummy_probability = @as(f64, @floatFromInt(entropy[5] % 16)) / 100.0, .dummy_probability = @as(f64, @floatFromInt(entropy[5] % 16)) / 100.0,
.dummy_distribution = if (entropy[6] % 2 == 0) .Uniform else .Bursty, .dummy_distribution = if (entropy[6] % 2 == 0) .Uniform else .Bursty,
.epoch_packet_count = 100 + (@as(u32, entropy[7]) * 4), // 100-1116 packets .epoch_packet_count = epoch_count,
}; };
} }