Linux 内核内置 C 库 nolibc 及其 RISC-V 架构支持分析

# 38e6304^:tools/testing/selftests/rcutorture/bin/mkinitrd.sh
#!/bin/sh
while :
do
        sleep 1000000
done

// 38e6304:tools/testing/selftests/rcutorture/bin/mkinitrd.sh
#include <unistd.h>

int main(int argc, int argv[])
{
        for (;;)
                sleep(1000*1000*1000); /* One gigasecond is ~30 years. */
        return 0;
}

$ gcc -s -static -Os -o init init.c
$ ls -lh init
-rwxrwxr-x 1 ubuntu ubuntu 780K 2月  10 15:07 init

$ echo "int main() { return sleep(3);}" | gcc -Os -nostdlib -include ../nolibc/nolibc.h -s
 -fno-exceptions -fno-asynchronous-unwind-tables -fno-unwind-tables  -lgcc -o sleep -xc -
$ ls -l sleep
-rwxr-xr-x 1 willy users 664 Aug 23 20:37 sleep

commit 66b6f755ad45d354c5b74abd258f67aa8b40b3c7
Author: Willy Tarreau <w@1wt.eu>
Date:   Sun Sep 9 13:26:04 2018 +0200

    rcutorture: Import a copy of nolibc

    This is a definition of the most common syscalls needed in minimalist
    init executables, allowing to statically build them with no external
    dependencies. It is sufficient in its current form to build rcutorture's
    init on x86_64, i386, arm, and arm64. Others have not been ported or
    tested. Updates may be found here :

         http://git.formilux.org/?p=people/willy/nolibc.git

    Signed-off-by: Willy Tarreau <w@1wt.eu>
    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

commit b94ec36896dafc0a12106b1536fe87f99e9a0c5d
Author: Willy Tarreau <w@1wt.eu>
Date:   Sun Sep 9 13:33:02 2018 +0200

    rcutorture: Make use of nolibc when available

    This reduces the size of the init executable from ~800 kB to ~800 bytes
    on x86_64. This is only implemented for x86_64, i386, arm and arm64.
    Others not tested.

    Signed-off-by: Willy Tarreau <w@1wt.eu>
    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/tools/testing/selftests/rcutorture/bin/mkinitrd.sh b/tools/testing/selftests/rcutorture/bin/mkinitrd.sh
index 56a56ea06983..da298394daa2 100755
--- a/tools/testing/selftests/rcutorture/bin/mkinitrd.sh
+++ b/tools/testing/selftests/rcutorture/bin/mkinitrd.sh
@@ -82,8 +82,10 @@ cd $D
 mkdir -p initrd
 cd initrd
 cat > init.c << '___EOF___'
+#ifndef NOLIBC
 #include <unistd.h>
 #include <sys/time.h>
+#endif

 volatile unsigned long delaycount;

@@ -113,7 +115,21 @@ int main(int argc, int argv[])
        return 0;
 }
 ___EOF___
-${CROSS_COMPILE}gcc -s -static -Os -o init init.c
+
+# build using nolibc on supported archs (smaller executable) and fall
+# back to regular glibc on other ones.
+if echo -e "#if __x86_64__||__i386__||__i486__||__i586__||__i686__" \
+           "||__ARM_EABI__||__aarch64__\nyes\n#endif" \
+   | ${CROSS_COMPILE}gcc -E -nostdlib -xc - \
+   | grep -q '^yes'; then
+       # architecture supported by nolibc
+        ${CROSS_COMPILE}gcc -fno-asynchronous-unwind-tables -fno-ident \
+               -nostdlib -include ../bin/nolibc.h -lgcc -s -static -Os \
+               -o init init.c
+else
+       ${CROSS_COMPILE}gcc -s -static -Os -o init init.c
+fi
+
 rm init.c
 echo "Done creating a statically linked C-language initrd"

commit 30ca20517ac136e63967396899af89f359f16f36
Author: Willy Tarreau <w@1wt.eu>
Date:   Sat Dec 29 19:04:53 2018 +0100

    tools headers: Move the nolibc header from rcutorture to tools/include/nolibc/

    As suggested by Ingo, this header file might benefit other tools than
    just rcutorture. For now it's quite limited, but is easy to extend, so
    exposing it into tools/include/nolibc/ will make it much easier to
    adopt by other tools.

    The mkinitrd.sh script in rcutorture was updated to use this new location.

    Cc: Ingo Molnar <mingo@kernel.org>
    Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
    Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Signed-off-by: Willy Tarreau <w@1wt.eu>
    Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>

$ git show v6.2-rc7:tools/include/nolibc/ | grep arch-
arch-aarch64.h
arch-arm.h
arch-i386.h
arch-mips.h
arch-riscv.h
arch-x86_64.h

$ git show v6.2-rc7:tools/include/nolibc/ | grep arch- | xargs -i wc -l tools/include/nolibc/{}
199 tools/include/nolibc/arch-aarch64.h
204 tools/include/nolibc/arch-arm.h
219 tools/include/nolibc/arch-i386.h
215 tools/include/nolibc/arch-mips.h
204 tools/include/nolibc/arch-riscv.h
215 tools/include/nolibc/arch-x86_64.h

$ cat tools/include/nolibc/arch-riscv.h | egrep -v '^[[:space:]]*$|^[[:space:]]*\\$' | wc -l
183

/* Syscalls for RISCV :
 *   - stack is 16-byte aligned
 *   - syscall number is passed in a7
 *   - arguments are in a0, a1, a2, a3, a4, a5
 *   - the system call is performed by calling ecall
 *   - syscall return comes in a0
 *   - the arguments are cast to long and assigned into the target
 *     registers which are then simply passed as registers to the asm code,
 *     so that we don't have to experience issues with register constraints.
 *
 * On riscv, select() is not implemented so we have to use pselect6().
 */
#define __ARCH_WANT_SYS_PSELECT6

#define my_syscall0(num)                                                      \
({                                                                            \
        register long _num  __asm__ ("a7") = (num);                           \
        register long _arg1 __asm__ ("a0");                                   \
                                                                              \
        __asm__  volatile (                                                   \
                "ecall\n\t"                                                   \
                : "=r"(_arg1)                                                 \
                : "r"(_num)                                                   \
                : "memory", "cc"                                              \
        );                                                                    \
        _arg1;                                                                \
})

#define my_syscall1(num, arg1)                                                \
({                                                                            \
        register long _num  __asm__ ("a7") = (num);                           \
        register long _arg1 __asm__ ("a0") = (long)(arg1);                    \
                                                                              \
        __asm__  volatile (                                                   \
                "ecall\n"                                                     \
                : "+r"(_arg1)                                                 \
                : "r"(_num)                                                   \
                : "memory", "cc"                                              \
        );                                                                    \
        _arg1;                                                                \
})

#if   __riscv_xlen == 64
#define PTRLOG "3"
#define SZREG  "8"
#elif __riscv_xlen == 32
#define PTRLOG "2"
#define SZREG  "4"
#endif

...

/* startup code */
__asm__ (".section .text\n"
    ".weak _start\n"
    "_start:\n"
    ".option push\n"
    ".option norelax\n"
    "lla   gp, __global_pointer$\n"
    ".option pop\n"
    "lw    a0, 0(sp)\n"          // argc (a0) was in the stack
    "add   a1, sp, "SZREG"\n"    // argv (a1) = sp
    "slli  a2, a0, "PTRLOG"\n"   // envp (a2) = SZREG*argc ...
    "add   a2, a2, "SZREG"\n"    //             + SZREG (skip null)
    "add   a2,a2,a1\n"           //             + argv
    "andi  sp,a1,-16\n"          // sp must be 16-byte aligned
    "call  main\n"               // main() returns the status code, we'll exit with it.
    "li a7, 93\n"                // NR_exit == 93
    "ecall\n"
    "");

$ grep "__NR_exit " -ur include/uapi/asm-generic/unistd.h
#define __NR_exit 93

// 切换板子
$ make BOARD=riscv64/virt

// 启用 nolibc 模式
$ export nolibc=1
$ make kernel nolibc_src=$PWD/src/examples/nolibc/hello.c
$ make boot
Starting kernel ...

Linux version 6.1.1 (ubuntu@linux-lab) (riscv64-linux-gnu-gcc (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0, GNU ld (GNU Binutils for Ubuntu) 2.34) #35 SMP Fri Feb 10 13:24:36 CST 2023
...
Freeing unused kernel image (initmem) memory: 2468K
Run /init as init process
Hello, nolibc!
reboot: System halted
ubuntu@linux-lab:/labs/linux-lab$

$ cat src/examples/nolibc/hello.c
#include <stdio.h>
#include <unistd.h>

int main(int argc, char *argv[])
{
	printf("Hello, nolibc!\n");

#ifdef NOLIBC
	reboot(LINUX_REBOOT_CMD_HALT);
#endif

	return 0;
}

$ ls -lh build/riscv64/virt/linux/v6.1.1/nolibc/initramfs/init
-rwxr-xr-x 1 ubuntu ubuntu 1.5K 2月  10 13:24 build/riscv64/virt/linux/v6.1.1/nolibc/initramfs/init