[libc][cmake][linux] require new LLVM_LIBC_USE_HOST_KERNEL_HEADERS or LIBC_KERNEL_HEADERS

When cross compiling, we DO NOT want to use the host's kernel headers. Adding
all of /usr/include via `-dirafter` also isn't very hermetic since it can pull
in more than just kernel headers. But peeking at /usr/include simplifies
setting up the libc, since then the kernel headers don't have to be built from
source. (Building kernel headers from source is quite trivial though).

We already support setting -DLIBC_KERNEL_HEADERS=/path/to/kernel/headers. Add a
new boolean cmake flag, LLVM_LIBC_USE_HOST_KERNEL_HEADERS, which indicates that
the user is opting into just using the hosts kernel headers. Existing host
builds may break and need to set this.

Setting LIBC_KERNEL_HEADERS currently produces a few unit test failures that I
still need to debug, but ideally users targeting linux will always pass
LIBC_KERNEL_HEADERS in the future.

Add some checks to ensure that at least one of these flags are set, that
they're mutually exclusive, that the headers are required when cross compiling,
and that the headers path is a directory.

Author: nickdesaulniers

libc/CMakeLists.txt

@@ -46,7 +46,11 @@ set(LIBC_BUILD_DIR ${CMAKE_CURRENT_BINARY_DIR})
 
 set(LIBC_ENABLE_USE_BY_CLANG OFF CACHE BOOL "Whether or not to place libc in a build directory findable by a just built clang")
 
-set(LIBC_KERNEL_HEADERS "/usr/include" CACHE STRING "Path to Linux kernel headers")
+# LLVM_LIBC_USE_HOST_KERNEL_HEADERS is brittle, and frowned upon (prefer
+# setting LIBC_KERNEL_HEADERS), but is simpler from a hello world linux host
+# build.
+set(LLVM_LIBC_USE_HOST_KERNEL_HEADERS OFF CACHE BOOL "Add /usr/include to the header seach path")
+set(LIBC_KERNEL_HEADERS "" CACHE STRING "Path to Linux kernel headers")
 
 # Defining a global namespace to enclose all libc functions.
 set(default_namespace "__llvm_libc")

libc/cmake/modules/LLVMLibCArchitectures.cmake

@@ -216,6 +216,27 @@ if (LIBC_TARGET_OS_IS_WINDOWS AND LLVM_LIBC_FULL_BUILD)
   message(FATAL_ERROR "Windows does not support full mode build.")
 endif ()
 
+if (LIBC_TARGET_OS_IS_LINUX)
+  if (NOT LLVM_LIBC_USE_HOST_KERNEL_HEADERS AND NOT LIBC_KERNEL_HEADERS STREQUAL "")
+    message(FATAL_ERROR "MUST specify either LIBC_KERNEL_HEADERS or LLVM_LIBC_USE_HOST_KERNEL_HEADERS")
+  endif()
+  if (LLVM_LIBC_USE_HOST_KERNEL_HEADERS AND NOT LIBC_KERNEL_HEADERS STREQUAL "")
+    message(FATAL_ERROR "LLVM_LIBC_USE_HOST_KERNEL_HEADERS and LIBC_USE_HOST_KERNEL_HEADERS are mutually exclusive")
+  endif()
+  if(LIBC_TARGET_TRIPLE AND LLVM_LIBC_USE_HOST_KERNEL_HEADERS)
+    # This is because the syscall numbers are frequently different between
+    # different target architectures. Code may compile, but you'll get spooky
+    # runtime failures.
+    message(FATAL_ERROR "LLVM_LIBC_USE_HOST_KERNEL_HEADERS should not be set when using LIBC_TARGET_TRIPLE, set LIBC_KERNEL_HEADERS instead")
+  endif()
+  if (NOT LIBC_KERNEL_HEADERS STREQUAL "" AND NOT IS_DIRECTORY LIBC_KERNEL_HEADERS)
+    message(FATAL_ERROR "LIBC_KERNEL_HEADERS should be a path to an existing directory")
+  endif()
+  if (LLVM_LIBC_USE_HOST_KERNEL_HEADERS)
+    set(LIBC_KERNEL_HEADERS "/usr/include" CACHE STRING "Path to Linux kernel headers" FORCE)
+  endif()
+endif()
+
 
 message(STATUS
         "Building libc for ${LIBC_TARGET_ARCHITECTURE} on ${LIBC_TARGET_OS} with

libc/docs/getting_started.rst

@@ -21,6 +21,7 @@ Install dependencies first:
     -DCMAKE_CXX_COMPILER=clang++ \
     -DCMAKE_C_COMPILER=clang \
     -DLLVM_LIBC_FULL_BUILD=ON \
+    -DLLVM_LIBC_USE_HOST_KERNEL_HEADERS=ON \
     -DLLVM_LIBC_INCLUDE_SCUDO=ON \
     -DCOMPILER_RT_BUILD_SCUDO_STANDALONE_WITH_LLVM_LIBC=ON \
     -DCOMPILER_RT_BUILD_GWP_ASAN=OFF \