ld: fix Mach-O X86_64_RELOC_SIGNED relocations

Fixes #1658.

R=rsc
CC=golang-dev
https://golang.org/cl/4344066

diff --git a/src/cmd/ld/ldmacho.c b/src/cmd/ld/ldmacho.c
index fbf2f1779b5a5b86797a241b991a8e5725d2f368..ad275dca8cadb34a560fb1074a7f51688eb8b4a8 100644 (file)
--- a/src/cmd/ld/ldmacho.c
+++ b/src/cmd/ld/ldmacho.c
@@ -422,6 +422,7 @@ void
 ldmacho(Biobuf *f, char *pkg, int64 len, char *pn)
 {
        int i, j, is64;
+       uint64 secaddr;
        uchar hdr[7*4], *cmdp;
        uchar tmp[4];
        uchar *dat;
@@ -754,8 +755,29 @@ ldmacho(Biobuf *f, char *pkg, int64 len, char *pn)
                        rp->siz = rel->length;
                        rp->type = 512 + (rel->type<<1) + rel->pcrel;
                        rp->off = rel->addr;
-                       
-                       rp->add = e->e32(s->p+rp->off);
+
+                       // Handle X86_64_RELOC_SIGNED referencing a section (rel->extrn == 0).
+                       if (thechar == '6' && rel->extrn == 0 && rel->type == 1) {
+                               // Calculate the addend as the offset into the section.
+                               //
+                               // The rip-relative offset stored in the object file is encoded
+                               // as follows:
+                               //    
+                               //    movsd     0x00000360(%rip),%xmm0
+                               //
+                               // To get the absolute address of the value this rip-relative address is pointing
+                               // to, we must add the address of the next instruction to it. This is done by
+                               // taking the address of the relocation and adding 4 to it (since the rip-relative
+                               // offset can at most be 32 bits long).  To calculate the offset into the section the
+                               // relocation is referencing, we subtract the vaddr of the start of the referenced
+                               // section found in the original object file.
+                               //
+                               // [For future reference, see Darwin's /usr/include/mach-o/x86_64/reloc.h]
+                               secaddr = c->seg.sect[rel->symnum-1].addr;
+                               rp->add = e->e32(s->p+rp->off) + rp->off + 4 - secaddr;
+                       } else
+                               rp->add = e->e32(s->p+rp->off);
+
                        // For i386 Mach-O PC-relative, the addend is written such that
                        // it *is* the PC being subtracted.  Use that to make
                        // it match our version of PC-relative.