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[scudo] Always zero on linux even if the memory cannot be released. #167788

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Merged
merged 3 commits into from
Nov 13, 2025
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Merged

[scudo] Always zero on linux even if the memory cannot be released. #167788

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613a09c
[scudo] Always zero on linux even if the memory cannot be released.
cferris1000 Nov 12, 2025
f2331c0
Add munlock call.
cferris1000 Nov 12, 2025
7da93ec
Fix formatting.
cferris1000 Nov 13, 2025
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7 changes: 6 additions & 1 deletion compiler-rt/lib/scudo/standalone/mem_map_linux.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -122,7 +122,12 @@ void MemMapLinux::setMemoryPermissionImpl(uptr Addr, uptr Size, uptr Flags) {
void MemMapLinux::releaseAndZeroPagesToOSImpl(uptr From, uptr Size) {
void *Addr = reinterpret_cast<void *>(From);

while (madvise(Addr, Size, MADV_DONTNEED) == -1 && errno == EAGAIN) {
int rc;
while ((rc = madvise(Addr, Size, MADV_DONTNEED)) == -1 && errno == EAGAIN) {
}
if (rc == -1) {
// If we can't madvies the memory, then we still need to zero it.
memset(Addr, 0, Size);
Comment on lines +128 to +130
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@ChiaHungDuan ChiaHungDuan Nov 13, 2025

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I was thinking if we want to check something like "errno == EPERM". It seems not making any difference to the problem but maybe it helps with some other cases. I guess not...

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@cferris1000 cferris1000 Nov 13, 2025

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Yeah, when I checked this the errno was EINVAL, which is not what I was expecting. It seems safer to just memset when it fails for whatever reason.

}
}

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46 changes: 46 additions & 0 deletions compiler-rt/lib/scudo/standalone/tests/map_test.cpp
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Expand Up @@ -14,6 +14,10 @@
#include <string.h>
#include <unistd.h>

#if SCUDO_LINUX
#include <sys/mman.h>
#endif

static const char *MappingName = "scudo:test";

TEST(ScudoMapTest, PageSize) {
Expand Down Expand Up @@ -89,3 +93,45 @@ TEST(ScudoMapTest, MapGrowUnmap) {
memset(reinterpret_cast<void *>(Q), 0xbb, PageSize);
MemMap.unmap();
}

// Verify that zeroing works properly.
TEST(ScudoMapTest, Zeroing) {
scudo::ReservedMemoryT ReservedMemory;
const scudo::uptr PageSize = scudo::getPageSizeCached();
const scudo::uptr Size = 3 * PageSize;
ReservedMemory.create(/*Addr=*/0U, Size, MappingName);
ASSERT_TRUE(ReservedMemory.isCreated());

scudo::MemMapT MemMap = ReservedMemory.dispatch(ReservedMemory.getBase(),
ReservedMemory.getCapacity());
EXPECT_TRUE(
MemMap.remap(MemMap.getBase(), MemMap.getCapacity(), MappingName));
unsigned char *Data = reinterpret_cast<unsigned char *>(MemMap.getBase());
memset(Data, 1U, MemMap.getCapacity());
// Spot check some values.
EXPECT_EQ(1U, Data[0]);
EXPECT_EQ(1U, Data[PageSize]);
EXPECT_EQ(1U, Data[PageSize * 2]);
MemMap.releaseAndZeroPagesToOS(MemMap.getBase(), MemMap.getCapacity());
EXPECT_EQ(0U, Data[0]);
EXPECT_EQ(0U, Data[PageSize]);
EXPECT_EQ(0U, Data[PageSize * 2]);

#if SCUDO_LINUX
// Now verify that if madvise fails, the data is still zeroed.
memset(Data, 1U, MemMap.getCapacity());
EXPECT_NE(-1, mlock(Data, MemMap.getCapacity()));

EXPECT_EQ(1U, Data[0]);
EXPECT_EQ(1U, Data[PageSize]);
EXPECT_EQ(1U, Data[PageSize * 2]);
MemMap.releaseAndZeroPagesToOS(MemMap.getBase(), MemMap.getCapacity());
EXPECT_EQ(0U, Data[0]);
EXPECT_EQ(0U, Data[PageSize]);
EXPECT_EQ(0U, Data[PageSize * 2]);

EXPECT_NE(-1, munlock(Data, MemMap.getCapacity()));
#endif

MemMap.unmap();
}
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