From 702fe7d60da4d38344f00200187fe1f0766bef8d Mon Sep 17 00:00:00 2001
From: Jamie Liu <jamieliu@google.com>
Date: Wed, 1 Sep 2021 13:21:40 -0700
Subject: Cache vdso.so's __kernel_rt_sigreturn location.

PiperOrigin-RevId: 394300607
---
 pkg/sentry/loader/loader.go | 10 +---------
 pkg/sentry/loader/vdso.go   | 40 ++++++++++++++++++----------------------
 2 files changed, 19 insertions(+), 31 deletions(-)

diff --git a/pkg/sentry/loader/loader.go b/pkg/sentry/loader/loader.go
index 6a356779c..2759ef71e 100644
--- a/pkg/sentry/loader/loader.go
+++ b/pkg/sentry/loader/loader.go
@@ -295,15 +295,7 @@ func Load(ctx context.Context, args LoadArgs, extraAuxv []arch.AuxEntry, vdso *V
 	m.SetEnvvEnd(sl.EnvvEnd)
 	m.SetAuxv(auxv)
 	m.SetExecutable(ctx, file)
-
-	symbolValue, err := getSymbolValueFromVDSO("rt_sigreturn")
-	if err != nil {
-		return 0, nil, "", syserr.NewDynamic(fmt.Sprintf("Failed to find rt_sigreturn in vdso: %v", err), syserr.FromError(err).ToLinux())
-	}
-
-	// Found rt_sigretrun.
-	addr := uint64(vdsoAddr) + symbolValue - vdsoPrelink
-	m.SetVDSOSigReturn(addr)
+	m.SetVDSOSigReturn(uint64(vdsoAddr) + vdsoSigreturnOffset - vdsoPrelink)
 
 	ac.SetIP(uintptr(loaded.entry))
 	ac.SetStack(uintptr(stack.Bottom))
diff --git a/pkg/sentry/loader/vdso.go b/pkg/sentry/loader/vdso.go
index 3abd2ee7d..bcee6aef6 100644
--- a/pkg/sentry/loader/vdso.go
+++ b/pkg/sentry/loader/vdso.go
@@ -19,7 +19,6 @@ import (
 	"debug/elf"
 	"fmt"
 	"io"
-	"strings"
 
 	"gvisor.dev/gvisor/pkg/abi"
 	"gvisor.dev/gvisor/pkg/context"
@@ -177,27 +176,6 @@ type VDSO struct {
 	phdrs []elf.ProgHeader `state:".([]elfProgHeader)"`
 }
 
-// getSymbolValueFromVDSO returns the specific symbol value in vdso.so.
-func getSymbolValueFromVDSO(symbol string) (uint64, error) {
-	f, err := elf.NewFile(bytes.NewReader(vdsodata.Binary))
-	if err != nil {
-		return 0, err
-	}
-	syms, err := f.Symbols()
-	if err != nil {
-		return 0, err
-	}
-
-	for _, sym := range syms {
-		if elf.ST_BIND(sym.Info) != elf.STB_LOCAL && sym.Section != elf.SHN_UNDEF {
-			if strings.Contains(sym.Name, symbol) {
-				return sym.Value, nil
-			}
-		}
-	}
-	return 0, fmt.Errorf("no %v in vdso.so", symbol)
-}
-
 // PrepareVDSO validates the system VDSO and returns a VDSO, containing the
 // param page for updating by the kernel.
 func PrepareVDSO(mfp pgalloc.MemoryFileProvider) (*VDSO, error) {
@@ -388,3 +366,21 @@ func (v *VDSO) Release(ctx context.Context) {
 	v.ParamPage.DecRef(ctx)
 	v.vdso.DecRef(ctx)
 }
+
+var vdsoSigreturnOffset = func() uint64 {
+	f, err := elf.NewFile(bytes.NewReader(vdsodata.Binary))
+	if err != nil {
+		panic(fmt.Sprintf("failed to parse vdso.so as ELF file: %v", err))
+	}
+	syms, err := f.Symbols()
+	if err != nil {
+		panic(fmt.Sprintf("failed to read symbols from vdso.so: %v", err))
+	}
+	const sigreturnSymbol = "__kernel_rt_sigreturn"
+	for _, sym := range syms {
+		if elf.ST_BIND(sym.Info) != elf.STB_LOCAL && sym.Section != elf.SHN_UNDEF && sym.Name == sigreturnSymbol {
+			return sym.Value
+		}
+	}
+	panic(fmt.Sprintf("no symbol %q in vdso.so", sigreturnSymbol))
+}()
-- 
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