[ELF2] Fix an assertion failure on 32-bit platform

[ikudrin]

Summary

As GOT section has lower address than PLT section, the Delta value is negative and should be calculated and stored in the signed variable.

[ikudrin]

The patch fixes the elf2/plt.s test failure on Windows (and probably on other platforms with 32-bit targets).

[• rafael]

How are you building it? The test already passes on a 32 bit linux build.

[ikudrin]

It fails on Windows (target triple i686-pc-win32) with MSVC 14.

A bit more explanation. The type uintprt_t in this situation is equivalent to 32-bit unsigned int. For example, the Delta var may have value 0xffffeffa. And when it's passed to the isInt<32> function, which parameter is 64-bit signed integer, it doesn't extend sign bit.

[ikudrin]

In Win32 build, uintptr_t is 32 bit unsigned integer. The isInt function
receives
64 bit signed integer as a parameter. A uint32 value converts to int64
without
sign bit extension, so isInt receives a positive value, which cannot be
represented
as signed 32 bit integer value.

I see nothing odd here.

[ikudrin]

Fixed in r248289

lld/trunk/ELF/OutputSections.cpp

template <class ELFT> void PltSection<ELFT>::writeTo(uint8_t *Buf) {
   ArrayRef<uint8_t> Jmp = {0xff, 0x25}; // jmpq *val(%rip)
   for (const SymbolBody *E : Entries) {
     uintptr_t InstPos = reinterpret_cast<uintptr_t>(Buf);
 
     memcpy(Buf, Jmp.data(), Jmp.size());
     Buf += Jmp.size();
 
     uintptr_t OffsetInPLT = (InstPos + 6) - Start;
-    uintptr_t Delta = GotSec.getEntryAddr(*E) - (this->getVA() + OffsetInPLT);
+    intptr_t Delta = static_cast<intptr_t>(GotSec.getEntryAddr(*E)) -
+                     static_cast<intptr_t>(this->getVA() + OffsetInPLT);
     assert(isInt<32>(Delta));
     support::endian::write32le(Buf, Delta);
     Buf += 4;
 
     *Buf = 0x90; // nop
     ++Buf;
     *Buf = 0x90; // nop
     ++Buf;