#Fixing asan misaligment issues

27 messages · Page 1 of 1 (latest)

celest tulip
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So I am pretty new to asan but I figured I would turn it on for my current project as it's actually supported and it's throwing a fair bit of warnings / errors. mainly in regard to memory loads / stores being misaligned

For example I get:

02:05:29    runtime error: store to misaligned address 0x020600000041 for type 'unsigned long', which requires 8 byte alignment
02:05:29    0x020600000041: note: pointer points here
02:05:29     00 00 00  01 be be be be be be be  be be be be be be be be  00 00 00 00 00 00 00 00  00 00 00 00 00

from the code ```cpp
template <class T>
inline void Write(const T& data)
{
EnsureHasRoom(sizeof(T));
((T)mWritePointer) = data;
mWritePointer += sizeof(T);

        mUsedSize = std::max(mUsedSize, GetPosition());
    }

where mWritePointer is an uint8_t*, I somewhat get what the issue is but I am really not sure how I would go about solving thse types of offisues
livid skiffBOT
#

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celest tulip
#

same with:

02:05:29     runtime error: member access within misaligned address 0x00020106886c for type '<redacted>', which requires 8 byte alignment
02:05:29    0x00020106886c: note: pointer points here
02:05:29      00 00 00 00 04 00 3c 00  6b 00 00 00 64 d4 5b 6e  ea 0a 00 00 01 00 00 00  76 8f 01 00 53 75 70 65

what would be from the folling lines:

        <redacted>* eventHeader = (<redacted>*)(inBlock.data() + readSize);
        readSize += eventHeader->size;
#

this mostly has to do with the fact I am working with buffers that essentially just store arbitrairy data that is either read or written to

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Fixing asan misaligment issues

south merlin
celest tulip
south merlin
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mWritePointer += reinterpret_cast<std::uintptr_t>(mWritePointer) % alignof(T);
celest tulip
#

right and same with reading

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you align it to the next multiple

south merlin
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this is kinda UB btw if you just reinterpret the pointer

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use std::memcpy

celest tulip
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ah, so doing a memcpy here would be fine?

south merlin
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std::memcpy(mWritePointer, &data, sizeof(data));
celest tulip
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that just removes any ub?

south merlin
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in this case yeah

celest tulip
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and then advance mWritePointer with sizeof(T)

south merlin
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you advance the pointer, then memcpy

celest tulip
#

uh right, so you still need to align it?

south merlin
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technically, you don't need to align it with std::memcpy but that's kinda dumb then since you can't access the object in the buffer

celest tulip
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mWritePointer += reinterpret_cast<std::uintptr_t>(mWritePointer) % alignof(T);
memcpy
mWritePointer += sizeof(T);
#

so essentially this?

south merlin
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if you just want it to be packed into the buffer, it's fine though

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you can just use std::memcpy in that case

south merlin
celest tulip
#

right 🤔 cause we generally pack into a buffer to serialize / compress it etc

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and it's just arbitrairy data / alignment