[MachO] Prevent overflow on 32-bit platforms when calculating load command offsets

[awilfox]

Summary

This prevents overflow which can lead to llvm-objdump incorrectly accepting a malformed MachO file. This in turn caused the LLVM test suite to segfault on 32-bit Linux/musl platforms (since pointers are allocated in high memory on musl).

Fixes GitHub #56746.

[awilfox]

This was my solution to the related changeset D138830. It simplifies the logic a lot compared to that revision while still ensuring no overflow during calculation. I did not see the other overflows so perhaps it may be better to merge the two in some way. I'm not sure how to do that in Phabricator.

[oontvoo]

In D138891#3955930, @awilfox wrote:

This was my solution to the related changeset D138830. It simplifies the logic a lot compared to that revision while still ensuring no overflow during calculation. I did not see the other overflows so perhaps it may be better to merge the two in some way. I'm not sure how to do that in Phabricator.

sorry, missed this revision/comment previously. I agree that this is simpler change, but I was hoping to fix the other potential overflows too . (Ack'ed that D138830. makes the code a bit ugly :\ .... open to other suggestions, tho)

[MaskRay]

LGTM.

llvm/lib/Object/MachOObjectFile.cpp

 getLoadCommandInfo(const MachOObjectFile &Obj, const char *Ptr,
                    uint32_t LoadCommandIndex) {
   if (auto CmdOrErr = getStructOrErr<MachO::load_command>(Obj, Ptr)) {
-    if (CmdOrErr->cmdsize + Ptr > Obj.getData().end())
+    uint64_t Offset = Ptr - Obj.getData().begin();
+    if (CmdOrErr->cmdsize + Offset > Obj.getData().size())
       return malformedError("load command " + Twine(LoadCommandIndex) +
                             " extends past end of file");
     if (CmdOrErr->cmdsize < 8)