Add AT_UID, AT_EUID, AT_GID, AT_EGID to aux vector.

With musl libc when these entries are missing from the aux vector
it's forcing libc.secure (effectively AT_SECURE). This mode prevents
RPATH and LD_LIBRARY_PATH from working.

https://git.musl-libc.org/cgit/musl/tree/ldso/dynlink.c#n1488
As the first entry is a mask of all the aux fields set:
https://git.musl-libc.org/cgit/musl/tree/ldso/dynlink.c#n187

PiperOrigin-RevId: 205284684
Change-Id: I04de7bab241043306b4f732306a81d74edfdff26

pkg/sentry/loader/BUILD

@@ -46,6 +46,7 @@ go_library(
         "//pkg/sentry/fs",
         "//pkg/sentry/fs/anon",
         "//pkg/sentry/fs/fsutil",
+        "//pkg/sentry/kernel/auth",
         "//pkg/sentry/limits",
         "//pkg/sentry/memmap",
         "//pkg/sentry/mm",

pkg/sentry/loader/loader.go

@@ -27,6 +27,7 @@ import (
 	"gvisor.googlesource.com/gvisor/pkg/sentry/arch"
 	"gvisor.googlesource.com/gvisor/pkg/sentry/context"
 	"gvisor.googlesource.com/gvisor/pkg/sentry/fs"
+	"gvisor.googlesource.com/gvisor/pkg/sentry/kernel/auth"
 	"gvisor.googlesource.com/gvisor/pkg/sentry/mm"
 	"gvisor.googlesource.com/gvisor/pkg/sentry/usermem"
 	"gvisor.googlesource.com/gvisor/pkg/syserror"
@@ -247,8 +248,14 @@ func Load(ctx context.Context, m *mm.MemoryManager, mounts *fs.MountNamespace, r
 		return 0, nil, "", err
 	}
 
-	// Add generic auxv entries
+	c := auth.CredentialsFromContext(ctx)
+
+	// Add generic auxv entries.
 	auxv := append(loaded.auxv, arch.Auxv{
+		arch.AuxEntry{linux.AT_UID, usermem.Addr(c.RealKUID.In(c.UserNamespace).OrOverflow())},
+		arch.AuxEntry{linux.AT_EUID, usermem.Addr(c.EffectiveKUID.In(c.UserNamespace).OrOverflow())},
+		arch.AuxEntry{linux.AT_GID, usermem.Addr(c.RealKGID.In(c.UserNamespace).OrOverflow())},
+		arch.AuxEntry{linux.AT_EGID, usermem.Addr(c.EffectiveKGID.In(c.UserNamespace).OrOverflow())},
 		arch.AuxEntry{linux.AT_CLKTCK, linux.CLOCKS_PER_SEC},
 		arch.AuxEntry{linux.AT_EXECFN, execfn},
 		arch.AuxEntry{linux.AT_RANDOM, random},