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cmd/asm, cmd/compile, runtime: add -spectre=ret mode

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This commit extends the -spectre flag to cmd/asm and adds
a new Spectre mitigation mode "ret", which enables the use
of retpolines.

Retpolines prevent speculation about the target of an indirect
jump or call and are described in more detail here:
https://support.google.com/faqs/answer/7625886

Change-Id: I4f2cb982fa94e44d91e49bd98974fd125619c93a
Reviewed-on: https://go-review.googlesource.com/c/go/+/222661
Reviewed-by: Keith Randall <khr@golang.org>
This commit is contained in:
Russ Cox 2020-01-17 13:54:30 -05:00
parent 877ef86bec
commit fc8a6336d1
13 changed files with 119 additions and 1 deletions
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11
src/cmd/internal/obj/x86/asm6.go
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@ -1875,6 +1875,17 @@ func span6(ctxt *obj.Link, s *obj.LSym, newprog obj.ProgAlloc) {
p.As = spadjop(ctxt, ASUBL, ASUBQ)
}
}
if ctxt.Retpoline && (p.As == obj.ACALL || p.As == obj.AJMP) && (p.To.Type == obj.TYPE_REG || p.To.Type == obj.TYPE_MEM) {
if p.To.Type != obj.TYPE_REG {
ctxt.Diag("non-retpoline-compatible: %v", p)
continue
}
p.To.Type = obj.TYPE_BRANCH
p.To.Name = obj.NAME_EXTERN
p.To.Sym = ctxt.Lookup("runtime.retpoline" + obj.Rconv(int(p.To.Reg)))
p.To.Reg = 0
p.To.Offset = 0
}
}
var count int64 // rough count of number of instructions