Fix deadlock in /proc/[pid]/fd/[num]

In order to resolve path names, fsSymlink.Readlink() may need to reenter
kernfs. Change the code so that kernfs.Inode.Readlink() is called without
locks and document the new contract.

PiperOrigin-RevId: 371770222

1 files changed, 8 insertions, 0 deletions

diff --git a/test/syscalls/linux/proc.cc b/test/syscalls/linux/proc.cc
index e95174291..143075e2d 100644
--- a/test/syscalls/linux/proc.cc
+++ b/test/syscalls/linux/proc.cc
@@ -2698,6 +2698,14 @@ TEST(Proc, Statfs) {
   EXPECT_EQ(st.f_namelen, NAME_MAX);
 }
 
+// Tests that /proc/[pid]/fd/[num] can resolve to a path inside /proc.
+TEST(Proc, ResolveSymlinkToProc) {
+  const auto proc = ASSERT_NO_ERRNO_AND_VALUE(Open("/proc/self/cmdline", 0));
+  const auto path = JoinPath("/proc/self/fd/", absl::StrCat(proc.get()));
+  const auto target = ASSERT_NO_ERRNO_AND_VALUE(ReadLink(path));
+  EXPECT_EQ(target, JoinPath("/proc/", absl::StrCat(getpid()), "/cmdline"));
+}
+
 }  // namespace
 }  // namespace testing
 }  // namespace gvisor