Tests: TestProcFs cannot assume stdin/stdout/stderr are the same

If someone runs the test with shell redirection going on, or in a way
that changes any of the standard file descriptors this assumption will
not hold. When running from a terminal normally, it is true however.

Instead, check that /proc/self/fd/[0,1,2] are symlinks, and can be
stat-d by verifying that both stat and lstat succeed, and give different
struct stat contents.

Tests/Kernel/TestProcFS.cpp

@@ -6,27 +6,45 @@
 
 #include <LibTest/TestCase.h>
 #include <cstring>
+#include <sys/stat.h>
 #include <unistd.h>
 
 TEST_CASE(test_process_fd_readlink)
 {
- char expected_link[MAXPATHLEN];
- char buf[MAXPATHLEN];
+ // Make sure that stdin, stdout and stderr are actually symlinks that point somewhere interesting
+ // Sadly we can't assume that they all point to the same file.
+ struct stat stat_buf = {};
+ struct stat lstat_buf = {};
+ auto rc = stat("/proc/self/fd/0", &stat_buf);
+ EXPECT_EQ(rc, 0);
+ rc = lstat("/proc/self/fd/0", &lstat_buf);
+ EXPECT_EQ(rc, 0);
+ EXPECT_NE(0, memcmp(&stat_buf, &lstat_buf, sizeof(struct stat)));
 
- // Read the symlink value for stdin, stdout and stderr
- auto link_length = readlink("/proc/self/fd/0", expected_link, sizeof(expected_link));
- expected_link[link_length] = '\0';
-
- link_length = readlink("/proc/self/fd/1", buf, sizeof(buf));
- buf[link_length] = '\0';
- EXPECT_EQ(0, strcmp(buf, expected_link));
+ stat_buf = {};
+ lstat_buf = {};
+ rc = stat("/proc/self/fd/1", &stat_buf);
+ EXPECT_EQ(rc, 0);
+ rc = lstat("/proc/self/fd/1", &lstat_buf);
+ EXPECT_EQ(rc, 0);
+ EXPECT_NE(0, memcmp(&stat_buf, &lstat_buf, sizeof(struct stat)));
 
- link_length = readlink("/proc/self/fd/2", buf, sizeof(buf));
- buf[link_length] = '\0';
- EXPECT_EQ(0, strcmp(buf, expected_link));
+ stat_buf = {};
+ lstat_buf = {};
+ rc = stat("/proc/self/fd/2", &stat_buf);
+ EXPECT_EQ(rc, 0);
+ rc = lstat("/proc/self/fd/2", &lstat_buf);
+ EXPECT_EQ(rc, 0);
+ EXPECT_NE(0, memcmp(&stat_buf, &lstat_buf, sizeof(struct stat)));
 
  // Create a new file descriptor that is a dup of 0 with various big values in order to reproduce issue #7820.
  // We should get the same link value for each fd that was duplicated.
+ char expected_link[MAXPATHLEN];
+ char buf[MAXPATHLEN];
+
+ // Read the symlink for stdin, stdout and stderr
+ auto link_length = readlink("/proc/self/fd/0", expected_link, sizeof(expected_link));
+ expected_link[link_length] = '\0';
 
  // 255 is the first broken file descriptor that was discovered and might be used by other software (e.g. bash)
  auto new_fd = dup2(0, 255);