LibC: Do not crash if `putenv` variable is invalid

Dr. POSIX says:

    Although the space used by string is no longer used once a new
    string which defines name is passed to putenv(), if any thread in
    the application has used getenv() to retrieve a pointer to this
    variable, it should not be freed by calling free(). If the changed
    environment variable is one known by the system (such as the locale
    environment variables) the application should never free the buffer
    used by earlier calls to putenv() for the same variable.

Applications _should_ not free the data passed to `putenv`, but they
_could_ in practice. I found that our Quake II port misbehaves in this
way, but does not crash on other platforms because glibc/musl `putenv`
does not assume that environment variables are correctly formatted.

The new behavior ignores environment variables without a '=' present,
and prevents excessively reading beyond the variable's name if the data
pointed to by the environment entry does not contain any null bytes.

With this change, our Quake II port no longer crashes when switching
from fullscreen to windowed mode.

Tests/LibC/CMakeLists.txt

@@ -2,6 +2,7 @@ set(TEST_SOURCES
     TestAbort.cpp
     TestAssert.cpp
     TestCType.cpp
+    TestEnvironment.cpp
     TestIo.cpp
     TestLibCExec.cpp
     TestLibCDirEnt.cpp

Tests/LibC/TestEnvironment.cpp

@@ -0,0 +1,59 @@
+/*
+ * Copyright (c) 2023, Jelle Raaijmakers <jelle@gmta.nl>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibTest/TestCase.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+static int putenv_from_stack(char const* environment_variable)
+{
+    char environment_buffer[32];
+    auto result = snprintf(environment_buffer, 31, "%s", environment_variable);
+    VERIFY(result > 0);
+    return putenv(environment_buffer);
+}
+
+static char const* getenv_with_overwritten_stack(char const* environment_variable_name)
+{
+    char environment_buffer[32];
+    memset(environment_buffer, ' ', 31);
+    environment_buffer[31] = 0;
+    return getenv(environment_variable_name);
+}
+
+TEST_CASE(putenv_overwrite_invalid_stack_value)
+{
+    // Write an environment variable using a stack value
+    auto result = putenv_from_stack("TESTVAR=123");
+    EXPECT_EQ(result, 0);
+
+    // Try to retrieve the variable after overwriting the stack
+    auto environment_variable = getenv_with_overwritten_stack("TESTVAR");
+    EXPECT_EQ(environment_variable, nullptr);
+
+    // Try to overwrite the variable now that it's zeroed out
+    char new_environment_value[32];
+    result = snprintf(new_environment_value, 31, "%s", "TESTVAR=456");
+    VERIFY(result > 0);
+    result = putenv(new_environment_value);
+    EXPECT_EQ(result, 0);
+
+    // Retrieve the variable and verify that it's set correctly
+    environment_variable = getenv("TESTVAR");
+    EXPECT_NE(environment_variable, nullptr);
+    EXPECT_EQ(strcmp(environment_variable, "456"), 0);
+
+    // Overwrite and retrieve it again to test correct search behavior for '='
+    char final_environment_value[32];
+    result = snprintf(final_environment_value, 31, "%s", "TESTVAR=789");
+    VERIFY(result > 0);
+    result = putenv(final_environment_value);
+    EXPECT_EQ(result, 0);
+    environment_variable = getenv("TESTVAR");
+    EXPECT_NE(environment_variable, nullptr);
+    EXPECT_EQ(strcmp(environment_variable, "789"), 0);
+}

Userland/Libraries/LibC/stdlib.cpp

@@ -414,9 +414,8 @@ char* getenv(char const* name)
         size_t varLength = eq - decl;
         if (vl != varLength)
             continue;
-        if (strncmp(decl, name, varLength) == 0) {
+        if (strncmp(decl, name, varLength) == 0)
             return eq + 1;
-        }
     }
     return nullptr;
 }
@@ -493,22 +492,23 @@ int serenity_putenv(char const* new_var, size_t length)
 int putenv(char* new_var)
 {
     char* new_eq = strchr(new_var, '=');
-
     if (!new_eq)
         return unsetenv(new_var);
 
-    auto new_var_len = new_eq - new_var;
+    auto new_var_name_len = new_eq - new_var;
     int environ_size = 0;
     for (; environ[environ_size]; ++environ_size) {
         char* old_var = environ[environ_size];
-        char* old_eq = strchr(old_var, '=');
-        VERIFY(old_eq);
-        auto old_var_len = old_eq - old_var;
+        auto old_var_name_max_length = strnlen(old_var, new_var_name_len);
+        char* old_eq = static_cast<char*>(memchr(old_var, '=', old_var_name_max_length + 1));
+        if (!old_eq)
+            continue; // possibly freed or overwritten value
 
-        if (new_var_len != old_var_len)
+        auto old_var_name_len = old_eq - old_var;
+        if (new_var_name_len != old_var_name_len)
             continue; // can't match
 
-        if (strncmp(new_var, old_var, new_var_len) == 0) {
+        if (strncmp(new_var, old_var, new_var_name_len) == 0) {
             free_environment_variable_if_needed(old_var);
             environ[environ_size] = new_var;
             return 0;
@@ -523,9 +523,8 @@ int putenv(char* new_var)
         return -1;
     }
 
-    for (int i = 0; environ[i]; ++i) {
+    for (int i = 0; environ[i]; ++i)
         new_environ[i] = environ[i];
-    }
 
     new_environ[environ_size] = new_var;
     new_environ[environ_size + 1] = nullptr;