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Fix undefined behavior in CSnapshotDelta::DiffItem

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Cast `int`s to `unsigned` before subtracting to ensure that integer wrapping is being used instead of causing undefined behavior. Same as in `UndiffItem`.

```
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior src/master/src/base/math.h:16:40 in
src/master/src/engine/shared/snapshot.cpp:206:21: runtime error: signed integer overflow: 256 - -2147483648 cannot be represented in type 'int'
    0 0x7650b7 in CSnapshotDelta::DiffItem(int const*, int const*, int*, int) src/master/src/engine/shared/snapshot.cpp:206:21
    1 0x765cea in CSnapshotDelta::CreateDelta(CSnapshot*, CSnapshot*, void*) src/master/src/engine/shared/snapshot.cpp:323:7
    2 0x51a0e2 in CServer::DoSnapshot() src/master/src/engine/server/server.cpp:964:36
    3 0x537486 in CServer::Run() src/master/src/engine/server/server.cpp:2818:6
    4 0x4feeb7 in main src/master/src/engine/server/main.cpp:190:21
    5 0x7fc51ec27d09 in __libc_start_main csu/../csu/libc-start.c:308:16
    6 0x4c3819 in _start (servers/DDNet-Server-ubsan+0x4c3819)

src/master/src/engine/shared/snapshot.cpp:206:21: runtime error: signed integer overflow: 1645289600 - -2139062144 cannot be represented in type 'int'
    0 0x7650b7 in CSnapshotDelta::DiffItem(int const*, int const*, int*, int) src/master/src/engine/shared/snapshot.cpp:206:21
    1 0x765cea in CSnapshotDelta::CreateDelta(CSnapshot*, CSnapshot*, void*) src/master/src/engine/shared/snapshot.cpp:323:7
    2 0x51a0e2 in CServer::DoSnapshot() src/master/src/engine/server/server.cpp:964:36
    3 0x537486 in CServer::Run() src/master/src/engine/server/server.cpp:2818:6
    4 0x4feeb7 in main src/master/src/engine/server/main.cpp:190:21
    5 0x7efd50c4ed09 in __libc_start_main csu/../csu/libc-start.c:308:16
    6 0x4c3819 in _start (servers/DDNet-Server-ubsan+0x4c3819)
```

See #6650.
This commit is contained in:
Robert Müller 2023-05-27 20:57:02 +02:00
parent ba7b82258e
commit 36dd5c5075
1 changed files with 2 additions and 1 deletions
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3
src/engine/shared/snapshot.cpp
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@ -203,7 +203,8 @@ int CSnapshotDelta::DiffItem(const int *pPast, const int *pCurrent, int *pOut, i
int Needed = 0;
while(Size)
{
*pOut = *pCurrent - *pPast;
// subtraction with wrapping by casting to unsigned
*pOut = (unsigned)*pCurrent - (unsigned)*pPast;
Needed |= *pOut;
pOut++;
pPast++;