Add Uid/Gid/Groups fields to VFS2 /proc/[pid]/status.

For comparison:

```
$ docker run --rm -it ubuntu:focal bash -c 'cat /proc/self/status'
Name:	cat
Umask:	0022
State:	R (running)
Tgid:	1
Ngid:	0
Pid:	1
PPid:	0
TracerPid:	0
Uid:	0	0	0	0
Gid:	0	0	0	0
FDSize:	64
Groups:
NStgid:	1
NSpid:	1
NSpgid:	1
NSsid:	1
VmPeak:	    2660 kB
VmSize:	    2660 kB
VmLck:	       0 kB
VmPin:	       0 kB
VmHWM:	     528 kB
VmRSS:	     528 kB
...

$ docker run --runtime=runsc-vfs2 --rm -it ubuntu:focal bash -c 'cat /proc/self/status'
Name:	cat
State:	R (running)
Tgid:	1
Pid:	1
PPid:	0
TracerPid:	0
Uid:	0	0	0	0
Gid:	0	0	0	0
FDSize:	4
Groups:
VmSize:	10708 kB
VmRSS:	3124 kB
VmData:	316 kB
...
```

Fixes #6374

PiperOrigin-RevId: 387465655

2 files changed, 39 insertions, 4 deletions

diff --git a/test/syscalls/linux/BUILD b/test/syscalls/linux/BUILD
index 7185df076..188c052b3 100644
--- a/test/syscalls/linux/BUILD
+++ b/test/syscalls/linux/BUILD
@@ -1750,6 +1750,7 @@ cc_binary(
         "//test/util:mount_util",
         "@com_google_absl//absl/container:node_hash_set",
         "@com_google_absl//absl/strings",
+        "@com_google_absl//absl/strings:str_format",
         "@com_google_absl//absl/synchronization",
         "@com_google_absl//absl/time",
         gtest,
diff --git a/test/syscalls/linux/proc.cc b/test/syscalls/linux/proc.cc
index 78aa73edc..8a4025fed 100644
--- a/test/syscalls/linux/proc.cc
+++ b/test/syscalls/linux/proc.cc
@@ -54,6 +54,8 @@
 #include "absl/strings/match.h"
 #include "absl/strings/numbers.h"
 #include "absl/strings/str_cat.h"
+#include "absl/strings/str_format.h"
+#include "absl/strings/str_join.h"
 #include "absl/strings/str_split.h"
 #include "absl/strings/string_view.h"
 #include "absl/synchronization/mutex.h"
@@ -88,6 +90,7 @@ using ::testing::Gt;
 using ::testing::HasSubstr;
 using ::testing::IsSupersetOf;
 using ::testing::Pair;
+using ::testing::StartsWith;
 using ::testing::UnorderedElementsAre;
 using ::testing::UnorderedElementsAreArray;
 
@@ -1622,10 +1625,41 @@ TEST(ProcPidStatusTest, HasBasicFields) {
 
     ASSERT_FALSE(status_str.empty());
     const auto status = ASSERT_NO_ERRNO_AND_VALUE(ParseProcStatus(status_str));
-    EXPECT_THAT(status, IsSupersetOf({Pair("Name", thread_name),
-                                      Pair("Tgid", absl::StrCat(tgid)),
-                                      Pair("Pid", absl::StrCat(tid)),
-                                      Pair("PPid", absl::StrCat(getppid()))}));
+    EXPECT_THAT(status, IsSupersetOf({
+                            Pair("Name", thread_name),
+                            Pair("Tgid", absl::StrCat(tgid)),
+                            Pair("Pid", absl::StrCat(tid)),
+                            Pair("PPid", absl::StrCat(getppid())),
+                        }));
+
+    if (!IsRunningWithVFS1()) {
+      uid_t ruid, euid, suid;
+      ASSERT_THAT(getresuid(&ruid, &euid, &suid), SyscallSucceeds());
+      gid_t rgid, egid, sgid;
+      ASSERT_THAT(getresgid(&rgid, &egid, &sgid), SyscallSucceeds());
+      std::vector<gid_t> supplementary_gids;
+      int ngids = getgroups(0, nullptr);
+      supplementary_gids.resize(ngids);
+      ASSERT_THAT(getgroups(ngids, supplementary_gids.data()),
+                  SyscallSucceeds());
+
+      EXPECT_THAT(
+          status,
+          IsSupersetOf(std::vector<
+                       ::testing::Matcher<std::pair<std::string, std::string>>>{
+              // gVisor doesn't support fsuid/gid, and even if it did there is
+              // no getfsuid/getfsgid().
+              Pair("Uid", StartsWith(absl::StrFormat("%d\t%d\t%d\t", ruid, euid,
+                                                     suid))),
+              Pair("Gid", StartsWith(absl::StrFormat("%d\t%d\t%d\t", rgid, egid,
+                                                     sgid))),
+              // ParseProcStatus strips leading whitespace for each value,
+              // so if the Groups line is empty then the trailing space is
+              // stripped.
+              Pair("Groups",
+                   StartsWith(absl::StrJoin(supplementary_gids, " "))),
+          }));
+    }
   });
 }