我在装有GLIBC 2.29和内核5.2.18-200.fc30.x86_64的Fedora 30机器上
$ rpm -qf /usr/lib64/libc.so.6
glibc-2.29-28.fc30.x86_64
override.c：
#define open Oopen
#define __xstat __Xxstat
#define _GNU_SOURCE
#include <dlfcn.h>
#include <stdio.h>
#include <sys/stat.h>
#include <syslog.h>
#include <errno.h>
#undef open
#undef __xstat
#ifndef DEBUG
#define DEBUG 1
#endif
#define LOG(fmt, ...) \
do { \
if (DEBUG) { \
int errno_ = errno; \
/* fprintf(stderr, "override|%s: " fmt, __func__, __VA_ARGS__); */ \
syslog(LOG_INFO | LOG_USER, "override|%s: " fmt, __func__, __VA_ARGS__); \
errno = errno_; \
} \
} while (0)
/* Function pointers to hold the value of the glibc functions */
static int (*real_open)(const char *str, int flags, mode_t mode);
static int (*real___xstat)(int ver, const char *str, struct stat *buf);
int open(const char *str, int flags, mode_t mode) {
LOG("%s\n", str);
real_open = dlsym(RTLD_NEXT, __func__);
return real_open(str, flags, mode);
}
int __xstat(int ver, const char *str, struct stat *buf) {
LOG("%s\n", str);
real___xstat = dlsym(RTLD_NEXT, __func__);
return real___xstat(ver, str, buf);
}
它在我能想到的所有情况下都有效，但不适用于此情况：
$ gcc -DDEBUG=1 -fPIC -shared -o liboverride.so override.c -ldl -Wall -Wextra -Werror
$ LD_PRELOAD=$PWD/liboverride.so bash -c "echo blah | xargs -I{} sh -c 'echo {} | rev'"
rev: stdin: Bad file descriptor
但是，如果我以syslog()的名义注释掉fprintf()，它会起作用：
$ gcc -DDEBUG=1 -fPIC -shared -o liboverride.so override.c -ldl -Wall -Wextra -Werror
$ LD_PRELOAD=$PWD/liboverride.so bash -c "echo blah | xargs -I{} sh -c 'echo {} | rev'"
override|open: /dev/tty
override|__xstat: /tmp/nwani_1587079071
override|__xstat: .
...
... yada ...
... yada ...
... yada ...
...
halb <----------------------------- !
...
... yada ...
... yada ...
... yada ...
override|__xstat: /usr/share/terminfo
所以，亲爱的朋友，我该如何调试为什么将syslog()结果转换为EBADF？
================================================ =========================>
更新：
$ LD_PRELOAD=$PWD/liboverride.so bash -c "echo blah | xargs -I{} sh -c 'echo {} | cat'"
cat替换为/usr/bin/cat，问题将消失。================================================ =========================>
更新：基于卡洛斯的回答，我在findutils（xargs）上运行了git bisect，发现（无意间）我的场景已添加功能：
commit 40cd25147b4461979c0d992299f2c101f9034f7a Author: Bernhard Voelker <[email protected]> Date: Tue Jun 6 08:19:29 2017 +0200 xargs: add -o, --open-tty option This option is available in the xargs implementation of FreeBSD, NetBSD, OpenBSD and in the Apple variant. Add it for compatibility. * xargs/xargs.c (open_tty): Add static flag for the new option. (longopts): Add member. (main): Handle the 'o' case in the getopt_long() loop. (prep_child_for_exec): Redirect stdin of the child to /dev/tty when the -o option is given. Furthermore, move the just-opened file descriptor to STDIN_FILENO. (usage): Document the new option. * bootstrap.conf (gnulib_modules): Add dup2. * xargs/xargs.1 (SYNOPSIS): Replace the too-long list of options by "[options]" - they are listed later anyway. (OPTIONS): Document the new option. (STANDARDS CONFORMANCE): Mention that the -o option is an extension. * doc/find.texi (xargs options): Document the new option. (Invoking the shell from xargs): Amend the explanation of the redirection example with a note about the -o option. (Viewing And Editing): Likewise. (Error Messages From xargs): Add the message when dup2() fails. (NEWS): Mention the new option. Fixes http://savannah.gnu.org/bugs/?51151我在装有GLIBC 2.29和内核5.2.18-200.fc30.x86_64 $ rpm -qf /usr/lib64/libc.so.6 glibc-2.29-28.fc30.x86_64 overlay.c的Fedora 30机器上： #define open Oopen #define __xstat __Xxstat #define ...
您覆盖的open和__xstat一定不能有正在运行的进程可以看到的副作用。
[没有进程期望open或__xstat关闭并重新打开编号最小的文件描述符，也不应该打开O_CLOEXEC，但这确实是syslog的工作，如果它发现日志记录套接字已失败。 >
解决方案是您必须在调用closelog之后再调用syslog，以避免任何副作用在过程中变得可见。