Change ARG_MAX/_SC_ARG_MAX back to a constant.

As per the lkml thread https://lkml.org/lkml/2017/11/1/946.

Bug: http://b/65818597
Test: ran tests
Change-Id: I7a0610e6903e6761f2b31416e2f5017bd7a60659

5 files changed, 16 insertions, 38 deletions

diff --git a/libc/bionic/sysconf.cpp b/libc/bionic/sysconf.cpp
index d45cb3874..2442fd9a7 100644
--- a/libc/bionic/sysconf.cpp
+++ b/libc/bionic/sysconf.cpp
@@ -52,9 +52,16 @@ long sysconf(int name) {
     // Things we actually have to calculate...
     //
     case _SC_ARG_MAX:
-      // Not a constant since Linux 2.6.23; see fs/exec.c for details.
-      // At least 32 pages, otherwise a quarter of the stack limit.
-      return MAX(__sysconf_rlimit(RLIMIT_STACK) / 4, _KERNEL_ARG_MAX);
+      // https://lkml.org/lkml/2017/11/15/813...
+      //
+      // I suspect a 128kB sysconf(_SC_ARG_MAX) is the sanest bet, simply
+      // because of that "conservative is better than aggressive".
+      //
+      // Especially since _technically_ we're still limiting things to that
+      // 128kB due to the single-string limit.
+      //
+      //               Linus
+      return ARG_MAX;
 
     case _SC_AVPHYS_PAGES:      return get_avphys_pages();
     case _SC_CHILD_MAX:         return __sysconf_rlimit(RLIMIT_NPROC);
diff --git a/libc/kernel/tools/defaults.py b/libc/kernel/tools/defaults.py
index b0fb95fb7..586cb0271 100644
--- a/libc/kernel/tools/defaults.py
+++ b/libc/kernel/tools/defaults.py
@@ -58,8 +58,6 @@ kernel_arch_token_replacements = {
 
 # Replace tokens in the output according to this mapping.
 kernel_token_replacements = {
-    # The kernel's ARG_MAX is actually the "minimum" maximum (see fs/exec.c).
-    "ARG_MAX": "_KERNEL_ARG_MAX",
     # The kernel usage of __unused for unused struct fields conflicts with the macro defined in <sys/cdefs.h>.
     "__unused": "__linux_unused",
     # The kernel usage of C++ keywords causes problems for C++ code so rename.
diff --git a/libc/kernel/uapi/linux/limits.h b/libc/kernel/uapi/linux/limits.h
index 41108f9b3..ad0e33ea2 100644
--- a/libc/kernel/uapi/linux/limits.h
+++ b/libc/kernel/uapi/linux/limits.h
@@ -20,7 +20,7 @@
 #define _LINUX_LIMITS_H
 #define NR_OPEN 1024
 #define NGROUPS_MAX 65536
-#define _KERNEL_ARG_MAX 131072
+#define ARG_MAX 131072
 #define LINK_MAX 127
 #define MAX_CANON 255
 #define MAX_INPUT 255
diff --git a/libc/upstream-freebsd/android/include/freebsd-compat.h b/libc/upstream-freebsd/android/include/freebsd-compat.h
index 8f0a30742..e646e23e1 100644
--- a/libc/upstream-freebsd/android/include/freebsd-compat.h
+++ b/libc/upstream-freebsd/android/include/freebsd-compat.h
@@ -50,6 +50,4 @@ void* reallocarray(void*, size_t, size_t);
 /* FreeBSD has this, but we can't really implement it correctly on Linux. */
 #define issetugid() 0
 
-#define ARG_MAX sysconf(_SC_ARG_MAX)
-
 #endif
diff --git a/tests/unistd_test.cpp b/tests/unistd_test.cpp
index 456ddded5..022da4db4 100644
--- a/tests/unistd_test.cpp
+++ b/tests/unistd_test.cpp
@@ -1017,36 +1017,11 @@ TEST(UNISTD_TEST, sysconf_SC_NPROCESSORS_ONLN) {
 }
 
 TEST(UNISTD_TEST, sysconf_SC_ARG_MAX) {
-  // Since Linux 2.6.23, ARG_MAX isn't a constant and depends on RLIMIT_STACK.
-
-  // Get our current limit, and set things up so we restore the limit.
-  rlimit rl;
-  ASSERT_EQ(0, getrlimit(RLIMIT_STACK, &rl));
-  uint64_t original_rlim_cur = rl.rlim_cur;
-  if (rl.rlim_cur == RLIM_INFINITY) {
-    rl.rlim_cur = 8 * 1024 * 1024; // Bionic reports unlimited stacks as 8MiB.
-  }
-  auto guard = android::base::make_scope_guard([&rl, original_rlim_cur]() {
-    rl.rlim_cur = original_rlim_cur;
-    ASSERT_EQ(0, setrlimit(RLIMIT_STACK, &rl));
-  });
-
-  // _SC_ARG_MAX should be 1/4 the stack size.
-  EXPECT_EQ(static_cast<long>(rl.rlim_cur / 4), sysconf(_SC_ARG_MAX));
-
-  // If you have a really small stack, the kernel still guarantees "32 pages" (fs/exec.c).
-  rl.rlim_cur = 1024;
-  rl.rlim_max = RLIM_INFINITY;
-  ASSERT_EQ(0, setrlimit(RLIMIT_STACK, &rl));
-
-  EXPECT_EQ(static_cast<long>(32 * sysconf(_SC_PAGE_SIZE)), sysconf(_SC_ARG_MAX));
-
-  // With a 128-page stack limit, we know exactly what _SC_ARG_MAX should be...
-  rl.rlim_cur = 128 * sysconf(_SC_PAGE_SIZE);
-  rl.rlim_max = RLIM_INFINITY;
-  ASSERT_EQ(0, setrlimit(RLIMIT_STACK, &rl));
-
-  EXPECT_EQ(static_cast<long>((128 * sysconf(_SC_PAGE_SIZE)) / 4), sysconf(_SC_ARG_MAX));
+  // https://lkml.org/lkml/2017/11/15/813.
+#if !defined(ARG_MAX)
+#define ARG_MAX 131072
+#endif
+  ASSERT_EQ(ARG_MAX, sysconf(_SC_ARG_MAX));
 }
 
 TEST(UNISTD_TEST, dup2_same) {