cmd/link: additional fixes for -newobj and "ld -r" ELF host objects

The previous fix for this issue (CL 208479) was not general enough;
this patch revises it to handle more cases.

The problem with the original fix was that once a sym.Symbol is
created for a given static symbol and given a bogus anonymous version
of -1, we hit problems if some other non-anonymous symbol (created by
host object loading) had relocations targeting the static symbol.

In this patch instead of assigning a fixed anonymous version of -1 to
such symbols, each time loader.Create is invoked we create a new
(unique) anonymous version for the sym.Symbol, then enter the result
into the loader's extStaticSyms map, permitting it to be found in
lookups when processing relocation targets.

NB: this code will hopefully get a lot simpler once we can move host
object loading away from early sym.Symbol creation.

Updates #35779.

Change-Id: I450ff577e17549025565d355d6707a2d28a5a617
Reviewed-on: https://go-review.googlesource.com/c/go/+/208778
Run-TryBot: Than McIntosh <thanm@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

diff --git a/src/cmd/link/internal/loader/loader.go b/src/cmd/link/internal/loader/loader.go
index 4e0dfb1e6435a74d843ea914f11fda06fdacd9a4..0adc395fef7c660b7089c20bfa873c9e4f62dfed 100644 (file)
--- a/src/cmd/link/internal/loader/loader.go
+++ b/src/cmd/link/internal/loader/loader.go
@@ -111,6 +111,8 @@ type Loader struct {
 
        Syms []*sym.Symbol // indexed symbols. XXX we still make sym.Symbol for now.
 
+       anonVersion int // most recently assigned ext static sym pseudo-version
+
        Reachable bitmap // bitmap of reachable symbols, indexed by global index
 
        // Used to implement field tracking; created during deadcode if
@@ -130,9 +132,6 @@ const (
        FlagStrictDups = 1 << iota
 )
 
-// anonVersion is used to tag symbols created by loader.Create.
-const anonVersion = -1
-
 func NewLoader(flags uint32) *Loader {
        nbuiltin := goobj2.NBuiltin()
        return &Loader{
@@ -350,7 +349,7 @@ func (l *Loader) resolve(r *oReader, s goobj2.SymRef) Sym {
 // This is more like Syms.ROLookup than Lookup -- it doesn't create
 // new symbol.
 func (l *Loader) Lookup(name string, ver int) Sym {
-       if ver >= sym.SymVerStatic {
+       if ver >= sym.SymVerStatic || ver < 0 {
                return l.extStaticSyms[nameVer{name, ver}]
        }
        return l.symsByName[ver][name]
@@ -846,13 +845,22 @@ func (l *Loader) ExtractSymbols(syms *sym.Symbols) {
        }
 
        // Add symbols to the ctxt.Syms lookup table. This explicitly
-       // skips things created via loader.Create (marked with
-       // anonVersion), since if we tried to add these we'd wind up with
-       // collisions.
+       // skips things created via loader.Create (marked with versions
+       // less than zero), since if we tried to add these we'd wind up
+       // with collisions. Along the way, update the version from the
+       // negative anon version to something larger than sym.SymVerStatic
+       // (needed so that sym.symbol.IsFileLocal() works properly).
+       anonVerReplacement := syms.IncVersion()
        for _, s := range l.Syms {
-               if s != nil && s.Name != "" && s.Version != anonVersion {
+               if s == nil {
+                       continue
+               }
+               if s.Name != "" && s.Version >= 0 {
                        syms.Add(s)
                }
+               if s.Version < 0 {
+                       s.Version = int16(anonVerReplacement)
+               }
        }
 }
 
@@ -985,11 +993,14 @@ func (l *Loader) LookupOrCreate(name string, version int, syms *sym.Symbols) *sy
        return s
 }
 
-// Create creates a symbol with the specified name, but does not
-// insert it into any lookup table (hence it is possible to create a
-// symbol name with name X when there is already an existing symbol
-// named X entered into the loader). This method is intended for
-// static/hidden symbols discovered while loading host objects.
+// Create creates a symbol with the specified name, returning a
+// sym.Symbol object for it. This method is intended for static/hidden
+// symbols discovered while loading host objects. We can see more than
+// one instance of a given static symbol with the same name/version,
+// so we can't add them to the lookup tables "as is". Instead assign
+// them fictitious (unique) versions, starting at -1 and decreasing by
+// one for each newly created symbol, and record them in the
+// extStaticSyms hash.
 func (l *Loader) Create(name string, syms *sym.Symbols) *sym.Symbol {
        i := l.max + 1
        l.max++
@@ -997,13 +1008,17 @@ func (l *Loader) Create(name string, syms *sym.Symbols) *sym.Symbol {
                l.extStart = i
        }
 
-       // Note the use of anonVersion -- this is to mark the symbol so that
-       // it can be skipped when ExtractSymbols is adding ext syms to the
-       // sym.Symbols hash.
-       l.extSyms = append(l.extSyms, nameVer{name, anonVersion})
+       // Assign a new unique negative version -- this is to mark the
+       // symbol so that it can be skipped when ExtractSymbols is adding
+       // ext syms to the sym.Symbols hash.
+       l.anonVersion--
+       ver := l.anonVersion
+       l.extSyms = append(l.extSyms, nameVer{name, ver})
        l.growSyms(int(i))
-       s := syms.Newsym(name, -1)
+       s := syms.Newsym(name, ver)
        l.Syms[i] = s
+       l.extStaticSyms[nameVer{name, ver}] = i
+
        return s
 }