kernel: add linux compatibility layer

It does not quite work yet, although binaries built for ArvernOS run on
Linux... "Just" need to make the opposite to happen now.

Makefile

@@ -159,6 +159,11 @@ ifeq ($(CONFIG_USE_FAKE_CLOCK), 1)
 	libk_c_files  += $(kernel_src_dir)/time/fake_clock.c
 endif
 
+ifeq ($(CONFIG_LINUX_COMPAT), 1)
+	KERNEL_CONFIG += -DCONFIG_LINUX_COMPAT
+	LIBC_CONFIG   += -DCONFIG_LINUX_COMPAT
+endif
+
 # This file exists in a Docker container because we copy it in `Dockerfile`.
 in_docker = $(wildcard /tmp/install-linux-deps)
 ifneq ($(in_docker),)
@@ -412,6 +417,7 @@ what: ## display some information about the current configuration
 	echo "OS_NAME : $(OS_NAME)"
 	echo "ARCH    : $(ARCH)"
 	echo ""
+	echo "CONFIG_LINUX_COMPAT   = $(CONFIG_LINUX_COMPAT)"
 	echo "CONFIG_SEMIHOSTING    = $(CONFIG_SEMIHOSTING)"
 	echo "CONFIG_USE_DLMALLOC   = $(CONFIG_USE_DLMALLOC)"
 	echo "CONFIG_USE_FAKE_CLOCK = $(CONFIG_USE_FAKE_CLOCK)"

Makefile-cfg.include

@@ -6,6 +6,8 @@ LLVM_SUFFIX ?=
 
 # When set to 1, enable the Undefined Behavior SANitizer.
 UBSAN                 ?=
+# When set to 1, enable Linux (binary) compatibility.
+CONFIG_LINUX_COMPAT   ?=
 # When set to 1, enable semi-hosting mode (QEMU, mainly).
 CONFIG_SEMIHOSTING    ?=
 # When set to 1, use dlmalloc for malloc/free/realloc (instead of liballoc).

include/libc/sys/linux_compat.h

@@ -0,0 +1,64 @@
+// https://chromium.googlesource.com/chromiumos/docs/+/HEAD/constants/syscalls.md
+#ifndef SYS_LINUX_COMPAT_H
+#define SYS_LINUX_COMPAT_H
+
+#ifdef __x86_64__
+
+#define SYSCALL_READ         0
+#define SYSCALL_WRITE        1
+#define SYSCALL_OPEN         2
+#define SYSCALL_CLOSE        3
+#define SYSCALL_FSTAT        5
+#define SYSCALL_LSEEK        8
+#define SYSCALL_GETPID       20
+#define SYSCALL_SOCKET       41
+#define SYSCALL_SENDTO       44
+#define SYSCALL_RECVFROM     45
+#define SYSCALL_EXECV        49
+#define SYSCALL_EXIT         60
+#define SYSCALL_GETTIMEOFDAY 96
+#define SYSCALL_REBOOT       169
+
+#elif __arm__
+
+#define SYSCALL_EXIT         1
+#define SYSCALL_READ         3
+#define SYSCALL_WRITE        4
+#define SYSCALL_OPEN         5
+#define SYSCALL_CLOSE        6
+#define SYSCALL_EXECV        11
+#define SYSCALL_LSEEK        19
+#define SYSCALL_GETPID       20
+#define SYSCALL_GETTIMEOFDAY 78
+#define SYSCALL_REBOOT       88
+#define SYSCALL_FSTAT        108
+#define SYSCALL_SOCKET       281
+#define SYSCALL_SENDTO       290
+#define SYSCALL_RECVFROM     292
+
+#elif __aarch64__
+
+#define SYSCALL_CLOSE        57
+#define SYSCALL_LSEEK        62
+#define SYSCALL_READ         63
+#define SYSCALL_WRITE        64
+#define SYSCALL_FSTAT        80
+#define SYSCALL_EXIT         93
+#define SYSCALL_REBOOT       142
+#define SYSCALL_GETTIMEOFDAY 169
+#define SYSCALL_GETPID       172
+#define SYSCALL_SOCKET       198
+#define SYSCALL_SENDTO       206
+#define SYSCALL_RECVFROM     207
+#define SYSCALL_EXECV        221
+// Not available on AArch64
+#define SYSCALL_OPEN         299
+
+#endif
+
+// Not available
+#define SYSCALL_TEST 298
+// Not available
+#define SYSCALL_GETHOSTBYNAME2 299
+
+#endif

include/libc/sys/syscall.h

@@ -11,6 +11,12 @@
 #include <sys/time.h>
 #include <sys/types.h>
 
+#ifdef CONFIG_LINUX_COMPAT
+
+#include <sys/linux_compat.h>
+
+#else
+
 #define SYSCALL_TEST           1
 #define SYSCALL_WRITE          2
 #define SYSCALL_READ           3
@@ -28,6 +34,8 @@
 #define SYSCALL_GETPID         15
 #define SYSCALL_EXIT           16
 
+#endif // CONFIG_LINUX_COMPAT
+
 #define SYSCALL_SET_ERRNO()                                                    \
   if (retval < 0) {                                                            \
     errno = -retval;                                                           \

src/kernel/sys/k_syscall.c

@@ -6,7 +6,7 @@
 
 typedef void (*syscall_ptr_t)(void);
 
-syscall_ptr_t syscall_handlers[200] = { (syscall_ptr_t)k_not_implemented };
+syscall_ptr_t syscall_handlers[300] = { (syscall_ptr_t)k_not_implemented };
 
 void syscall_init()
 {

src/userland/Makefile.include

@@ -85,6 +85,10 @@ ifeq ($(UBSAN), 1)
 	CFLAGS += -fsanitize=undefined
 endif
 
+ifeq ($(CONFIG_LINUX_COMPAT), 1)
+	CFLAGS += -DCONFIG_LINUX_COMPAT
+endif
+
 # This file exists in a Docker container because we copy it in `Dockerfile`.
 in_docker = $(wildcard /tmp/install-linux-deps)
 ifneq ($(in_docker),)