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cmd/internal/ld: do not depend on local symbols to read a type's gcdata

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We already read the address of a gcmask/gcprog out of the type data, but I
didn't know how many bytes to read. But it turns out that it's easy to
calculate, so change to do that. This means that we no longer depend on the
local symbols being present, allowing me to strip the shared libraries for
distribution and make them a lot smaller.

As a bonus, this makes LSym another 24 bytes smaller, down to 296 bytes now.

Change-Id: I379d359e28d63afae6753efd23efdf1fbb716992
Reviewed-on: https://go-review.googlesource.com/10377
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
This commit is contained in:
Michael Hudson-Doyle 2015-05-25 16:13:50 +12:00 • committed by Ian Lance Taylor
parent bcc1870fcf
commit c949cff6a7
6 changed files with 64 additions and 53 deletions
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49
src/cmd/link/internal/ld/lib.go
View file

@ -1178,7 +1178,7 @@ func readelfsymboldata(f *elf.File, sym *elf.Symbol) []byte {
if sect.Type != elf.SHT_PROGBITS && sect.Type != elf.SHT_NOTE {
Diag("reading %s from non-data section", sym.Name)
}
n, err := sect.ReadAt(data, int64(sym.Value-sect.Offset))
n, err := sect.ReadAt(data, int64(sym.Value-sect.Addr))
if uint64(n) != sym.Size {
Diag("reading contents of %s: %v", sym.Name, err)
}
@ -1265,7 +1265,6 @@ func ldshlibsyms(shlib string) {
Diag("cannot open shared library: %s", libpath)
return
}
defer f.Close()
hash, err := readnote(f, ELF_NOTE_GO_NAME, ELF_NOTE_GOABIHASH_TAG)
if err != nil {
@ -1280,33 +1279,15 @@ func ldshlibsyms(shlib string) {
}
deps := strings.Split(string(depsbytes), "\n")
syms, err := f.Symbols()
syms, err := f.DynamicSymbols()
if err != nil {
Diag("cannot read symbols from shared library: %s", libpath)
return
}
// If a package has a global variable of a type defined in another shared
// library, we need to know the gcmask used by the type, if any. To support
// this, we read all the runtime.gcbits.* symbols, keep a map of address to
// gcmask, and after we're read all the symbols, read the addresses of the
// gcmasks symbols out of the type data to look up the gcmask for each type.
// This depends on the fact that the runtime.gcbits.* symbols are local (so
// the address is actually present in the type data and we don't have to
// search all relocations to find the ones which correspond to gcmasks) and
// also that the shared library we are linking against has not had the symbol
// table removed.
gcmasks := make(map[uint64][]byte)
types := []*LSym{}
for _, s := range syms {
if elf.ST_TYPE(s.Info) == elf.STT_NOTYPE || elf.ST_TYPE(s.Info) == elf.STT_SECTION {
continue
}
if strings.HasPrefix(s.Name, "runtime.gcbits.") {
gcmasks[s.Value] = readelfsymboldata(f, &s)
}
if elf.ST_BIND(s.Info) != elf.STB_GLOBAL {
continue
}
lsym := Linklookup(Ctxt, s.Name, 0)
if lsym.Type != 0 && lsym.Type != obj.SDYNIMPORT && lsym.Dupok == 0 {
Diag(
@ -1315,29 +1296,17 @@ func ldshlibsyms(shlib string) {
}
lsym.Type = obj.SDYNIMPORT
lsym.ElfType = elf.ST_TYPE(s.Info)
lsym.File = libpath
if strings.HasPrefix(lsym.Name, "type.") {
if f.Sections[s.Section].Type == elf.SHT_PROGBITS {
if s.Section != elf.SHN_UNDEF {
// Set .File for the library that actually defines the symbol.
lsym.File = libpath
// The decodetype_* functions in decodetype.go need access to
// the type data.
if strings.HasPrefix(lsym.Name, "type.") && !strings.HasPrefix(lsym.Name, "type..") {
lsym.P = readelfsymboldata(f, &s)
}
if !strings.HasPrefix(lsym.Name, "type..") {
types = append(types, lsym)
}
}
}
for _, t := range types {
if decodetype_noptr(t) != 0 || decodetype_usegcprog(t) != 0 {
continue
}
addr := decodetype_gcprog_shlib(t)
tgcmask, ok := gcmasks[addr]
if !ok {
Diag("bits not found for %s at %d", t.Name, addr)
}
t.gcmask = tgcmask
}
// We might have overwritten some functions above (this tends to happen for the
// autogenerated type equality/hashing functions) and we don't want to generated
// pcln table entries for these any more so unstitch them from the Textp linked
@ -1365,7 +1334,7 @@ func ldshlibsyms(shlib string) {
Ctxt.Etextp = last
}
Ctxt.Shlibs = append(Ctxt.Shlibs, Shlib{Path: libpath, Hash: hash, Deps: deps})
Ctxt.Shlibs = append(Ctxt.Shlibs, Shlib{Path: libpath, Hash: hash, Deps: deps, File: f})
}
func mywhatsys() {