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AK: Don't read past the end in BitmapView::count_in_range()

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The current code is factored such that reads to the entirety of the last
byte should be dropped. This was relying on the fact that last would be
one past the end in that case. Instead of actually reading that byte
when it's completely out of bounds of the bitmask, just skip reads that
would be invalid. Add more tests to make sure that the behavior is
correct for byte aligned reads of byte aligned bitmaps.
This commit is contained in:
Andrew Kaster 2021-05-15 00:12:50 -06:00 committed by Linus Groh
parent b9d65da5c8
commit 73adbb319c
Notes: sideshowbarker 2024-07-18 17:59:48 +09:00 
Author: https://github.com/ADKaster
Commit: https://github.com/SerenityOS/serenity/commit/73adbb319ca
Pull-request: https://github.com/SerenityOS/serenity/pull/7157
Issue: https://github.com/SerenityOS/serenity/issues/7073
Reviewed-by: https://github.com/linusg
Reviewed-by: https://github.com/metmo
2 changed files with 35 additions and 4 deletions
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11
AK/BitmapView.h
View file

@ -52,7 +52,7 @@ public:
return 0;
static const u8 bitmask_first_byte[8] = { 0xFF, 0xFE, 0xFC, 0xF8, 0xF0, 0xE0, 0xC0, 0x80 };
static const u8 bitmask_last_byte[8] = { 0x0, 0x1, 0x3, 0x7, 0xF, 0x1F, 0x3F, 0x7F };
static const u8 bitmask_last_byte[8] = { 0x00, 0x1, 0x3, 0x7, 0xF, 0x1F, 0x3F, 0x7F };
size_t count;
const u8* first = &m_data[start / 8];
@ -64,9 +64,12 @@ public:
count = __builtin_popcount(byte);
} else {
count = __builtin_popcount(byte);
byte = *last;
byte &= bitmask_last_byte[(start + len) % 8];
count += __builtin_popcount(byte);
// Don't access *last if it's out of bounds
if (last < &m_data[size_in_bytes()]) {
byte = *last;
byte &= bitmask_last_byte[(start + len) % 8];
count += __builtin_popcount(byte);
}
if (++first < last) {
const u32* ptr32 = (const u32*)(((FlatPtr)first + sizeof(u32) - 1) & ~(sizeof(u32) - 1));
if ((const u8*)ptr32 > last)

28
Tests/AK/TestBitmap.cpp
View file

@ -248,3 +248,31 @@ TEST_CASE(count_in_range)
test_with_value(true);
test_with_value(false);
}
TEST_CASE(byte_aligned_access)
{
{
Bitmap bitmap(16, true);
EXPECT_EQ(bitmap.count_in_range(0, 16, true), 16u);
EXPECT_EQ(bitmap.count_in_range(8, 8, true), 8u);
EXPECT_EQ(bitmap.count_in_range(0, 8, true), 8u);
EXPECT_EQ(bitmap.count_in_range(4, 8, true), 8u);
}
{
Bitmap bitmap(16, false);
bitmap.set_range(4, 8, true);
EXPECT_EQ(bitmap.count_in_range(0, 16, true), 8u);
EXPECT_EQ(bitmap.count_in_range(8, 8, true), 4u);
EXPECT_EQ(bitmap.count_in_range(0, 8, true), 4u);
EXPECT_EQ(bitmap.count_in_range(4, 8, true), 8u);
}
{
Bitmap bitmap(8, false);
bitmap.set(2, true);
bitmap.set(4, true);
EXPECT_EQ(bitmap.count_in_range(0, 2, true), 0u);
EXPECT_EQ(bitmap.count_in_range(0, 4, true), 1u);
EXPECT_EQ(bitmap.count_in_range(0, 8, true), 2u);
EXPECT_EQ(bitmap.count_in_range(4, 4, true), 1u);
}
}