Re: [RFC PATCH v1 0/8] nolibc signal handling support
From: Ammar Faizi
Date: Thu Dec 22 2022 - 08:46:43 EST
On Thu, 22 Dec 2022 05:34:52 +0100, Willy Tarreau wrote:
> This one relies on /proc/self/auxv, but we'll quickly run into a
> chicken-and-egg situation given that nolibc is used by init programs
> that mount /proc. Instead I think that we should modify the _start
> code to retrieve the auxv at startup and store it somewhere. This
> "somewhere" is not yet defined, but I'm thinking that it could
> deserve reserving some room in the stack to store some nolibc-defined
> information (possibly even a copy of a pointer to environ and/or errno)
> and figure a reliable and simple way to access this. Note that one way
> could also be to know that it's after the NULL that follows envp, and
> to start from environ. In fact there are plenty of ways and we just
> need to decide what's the least ugly. But once we have access to the
> auxv from the process, then we could implement a getauxval() function
> to retrieve the information you need for getpagesize().
Thanks for the great feedback!
I agree with following the @envp pointer to get the auxv. I was
trying to wire up a new function '__start' (with double underscores)
written in C that accepts @argc, @argv and @envp. Then it calls 'main'.
Then we call '__start' instead of 'main' from '_start'. This way, we
can arrange nolibc-defined data without touching Assembly much in
'__start' (before main).
But then I noticed that it wouldn't work because we may have users
who define the 'main' function differently, e.g.:
int main(void);
int main(int argc, char **argv);
int main(int argc, char **argv, char **envp);
So '__start' can't call main. We still need to call the main from the
inline Assembly (from '_start').
Just a quick dirty patch to get getauxval() works on x86-64 below.
This needs more work, but at least something like this for starting:
$ ./nolibc-test
Running test 'syscall'
AT_SYSINFO_EHDR = 140737354125312
AT_HWCAP = 3219913727
AT_PAGESZ = 4096
AT_CLKTCK = 100
AT_PHDR = 4194368
AT_PHENT = 56
AT_PHNUM = 9
AT_BASE = 0
AT_FLAGS = 0
AT_ENTRY = 4199128
AT_UID = 0
AT_EUID = 0
AT_GID = 0
AT_EGID = 0
AT_SECURE = 0
AT_RANDOM = 140737488349065
AT_EXECFN = 140737488351210
AT_PLATFORM = 140737488349081
I'll probably only spare more intensive time on this after the holiday
season ends. I still check in email, but the activity will be sparse.
Signed-off-by: [Do not commit]
---
base-commit: caf5c36025ec9395c8d7c78957b016a284812d23 ("srcu: Update comment after the index flip")
diff --git a/tools/include/nolibc/arch-x86_64.h b/tools/include/nolibc/arch-x86_64.h
index 0e1e9eb8545d..5ee945370ff5 100644
--- a/tools/include/nolibc/arch-x86_64.h
+++ b/tools/include/nolibc/arch-x86_64.h
@@ -199,17 +199,49 @@ struct sys_stat_struct {
*/
__asm__ (".section .text\n"
".weak _start\n"
"_start:\n"
"pop %rdi\n" // argc (first arg, %rdi)
"mov %rsp, %rsi\n" // argv[] (second arg, %rsi)
"lea 8(%rsi,%rdi,8),%rdx\n" // then a NULL then envp (third arg, %rdx)
"xor %ebp, %ebp\n" // zero the stack frame
"and $-16, %rsp\n" // x86 ABI : esp must be 16-byte aligned before call
+ "push %rdi\n" // Save argc
+ "push %rsi\n" // Save arg
+ "push %rdx\n" // Save envp.
+ "push %rcx\n" // Keep the 16-byte alignment
+ "call __start\n" // Save environ and auxv
+ "pop %rcx\n" // Restore alignment
+ "pop %rdx\n" // Restore envp
+ "pop %rsi\n" // Restore argv
+ "pop %rdi\n" // Restore argc
"call main\n" // main() returns the status code, we'll exit with it.
"mov %eax, %edi\n" // retrieve exit code (32 bit)
"mov $60, %eax\n" // NR_exit == 60
"syscall\n" // really exit
"hlt\n" // ensure it does not return
"");
+struct __nolibc_internal {
+ char **envp;
+ unsigned long *auxv;
+};
+
+static struct __nolibc_internal __nolibc_internal;
+
+/*
+ * Mark this __used__ to avoid being optimized away.
+ * Reason: Called from inline Assembly.
+ */
+static __attribute__((__used__))
+void __start(int argc, char **argv, char **envp)
+{
+ char **p = envp;
+
+ while (*p)
+ p++;
+
+ __nolibc_internal.auxv = (unsigned long *)++p;
+ __nolibc_internal.envp = envp;
+}
+
#endif // _NOLIBC_ARCH_X86_64_H
diff --git a/tools/include/nolibc/sys.h b/tools/include/nolibc/sys.h
index 3db1dd8c74ee..04bff724e056 100644
--- a/tools/include/nolibc/sys.h
+++ b/tools/include/nolibc/sys.h
@@ -12,18 +12,19 @@
/* system includes */
#include <asm/unistd.h>
#include <asm/signal.h> // for SIGCHLD
#include <asm/ioctls.h>
#include <asm/mman.h>
#include <linux/fs.h>
#include <linux/loop.h>
#include <linux/time.h>
+#include <linux/auxvec.h>
#include "arch.h"
#include "errno.h"
#include "types.h"
/* Functions in this file only describe syscalls. They're declared static so
* that the compiler usually decides to inline them while still being allowed
* to pass a pointer to one of their instances. Each syscall exists in two
@@ -379,18 +380,49 @@ int fsync(int fd)
if (ret < 0) {
SET_ERRNO(-ret);
ret = -1;
}
return ret;
}
+/*
+ * On success, getauxval() returns the value corresponding to type.
+ * If type is not found, 0 is returned.
+ *
+ * unsigned long getauxval(unsigned long type);
+ */
+
+static __attribute__((unused))
+unsigned long getauxval(unsigned long type)
+{
+ unsigned long *auxv = __nolibc_internal.auxv;
+
+ if (__builtin_expect(!auxv, 0))
+ return 0;
+
+ while (1) {
+ if (!auxv[0] && !auxv[1])
+ /*
+ * We've reached the end of auxv.
+ */
+ return 0;
+
+ if (auxv[0] == type)
+ return auxv[1];
+
+ auxv += 2;
+ }
+ __builtin_unreachable();
+}
+
+
/*
* int getdents64(int fd, struct linux_dirent64 *dirp, int count);
*/
static __attribute__((unused))
int sys_getdents64(int fd, struct linux_dirent64 *dirp, int count)
{
return my_syscall3(__NR_getdents64, fd, dirp, count);
}
diff --git a/tools/testing/selftests/nolibc/nolibc-test.c b/tools/testing/selftests/nolibc/nolibc-test.c
index f14f5076fb6d..bca29a952c69 100644
--- a/tools/testing/selftests/nolibc/nolibc-test.c
+++ b/tools/testing/selftests/nolibc/nolibc-test.c
@@ -436,30 +436,60 @@ int test_getdents64(const char *dir)
ret = getdents64(fd, (void *)buffer, sizeof(buffer));
err = errno;
close(fd);
errno = err;
return ret;
}
+static void test_getauxval(void)
+{
+ #define PRINT_AUXVAL(KEY) \
+ do { \
+ printf(#KEY " = %lu\n", getauxval(KEY)); \
+ } while (0)
+
+ PRINT_AUXVAL(AT_SYSINFO_EHDR);
+ PRINT_AUXVAL(AT_HWCAP);
+ PRINT_AUXVAL(AT_PAGESZ);
+ PRINT_AUXVAL(AT_CLKTCK);
+ PRINT_AUXVAL(AT_PHDR);
+ PRINT_AUXVAL(AT_PHENT);
+ PRINT_AUXVAL(AT_PHNUM);
+ PRINT_AUXVAL(AT_BASE);
+ PRINT_AUXVAL(AT_FLAGS);
+ PRINT_AUXVAL(AT_ENTRY);
+ PRINT_AUXVAL(AT_UID);
+ PRINT_AUXVAL(AT_EUID);
+ PRINT_AUXVAL(AT_GID);
+ PRINT_AUXVAL(AT_EGID);
+ PRINT_AUXVAL(AT_SECURE);
+ PRINT_AUXVAL(AT_RANDOM);
+ PRINT_AUXVAL(AT_EXECFN);
+ PRINT_AUXVAL(AT_PLATFORM);
+ exit(0);
+}
+
/* Run syscall tests between IDs <min> and <max>.
* Return 0 on success, non-zero on failure.
*/
int run_syscall(int min, int max)
{
struct stat stat_buf;
int proc;
int test;
int tmp;
int ret = 0;
void *p1, *p2;
+ test_getauxval();
+
/* <proc> indicates whether or not /proc is mounted */
proc = stat("/proc", &stat_buf) == 0;
for (test = min; test >= 0 && test <= max; test++) {
int llen = 0; // line length
/* avoid leaving empty lines below, this will insert holes into
* test numbers.
*/
--
Ammar Faizi