Please let me know what are the _nocancel() system calls (e.g. __pwrite_nocancel(), and whether there is a way to create an LD_PRELOAD library to intercept those calls. Here's a bit of the background:
I'm investigating functionality of an Oracle database, and would like to add a small shim layer using LD_PRELOAD to capture information about the call in user space. I'm aware of other ways of capturing this information using system tap, but using LD_PRELOAD is a hard requirement from the customer.
strace shows that this particular process is calling pwrite() repeatedly; similarly, the pstack stack trace shows that __pwrite_nocancel() is being called as the last entry on the stack. I tried reproducing my own __libc_pwrite() function, and declaring
extern ssize_t pwrite(int fd, const void *buf, size_t numBytes, off_t offset)__attribute__((weak, alias ( "__libc_pwrite")));
but when I link the library and run nm -a |grep pwrite, I get this:
000000000006c190 T __libc_pwrite
000000000006c190 W pwrite
in contrast, nm -a /lib64/libpthread.so.0 |grep pwrite gives the following:
000000000000eaf0 t __libc_pwrite
000000000000eaf0 t __libc_pwrite64
000000000000eaf0 W pwrite
000000000000eaf0 t __pwrite
000000000000eaf0 W pwrite64
000000000000eaf0 W __pwrite64
0000000000000000 a pwrite64.c
000000000000eaf9 t __pwrite_nocancel
I've noticed that the _nocancel version is only 9 bytes ahead of the __pwrite, but looking at the source code, I'm unsure as to where it's being created:
/* Copyright (C) 1997, 1998, 2000, 2002, 2003, 2004, 2006
Free Software Foundation, Inc.
This file is part of the GNU C Library.
Contributed by Ulrich Drepper <drepper@cygnus.com>, 1997.
The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
The GNU C Library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with the GNU C Library; if not, see
<http://www.gnu.org/licenses/>. */
#include <assert.h>
#include <errno.h>
#include <unistd.h>
#include <endian.h>
#include <sysdep-cancel.h>
#include <sys/syscall.h>
#include <bp-checks.h>
#include <kernel-features.h>
#ifdef __NR_pwrite64 /* Newer kernels renamed but it's the same. */
# ifdef __NR_pwrite
# error "__NR_pwrite and __NR_pwrite64 both defined???"
# endif
# define __NR_pwrite __NR_pwrite64
#endif
#if defined __NR_pwrite || __ASSUME_PWRITE_SYSCALL > 0
# if __ASSUME_PWRITE_SYSCALL == 0
static ssize_t __emulate_pwrite (int fd, const void *buf, size_t count,
off_t offset) internal_function;
# endif
ssize_t
__libc_pwrite (fd, buf, count, offset)
int fd;
const void *buf;
size_t count;
off_t offset;
{
ssize_t result;
if (SINGLE_THREAD_P)
{
/* First try the syscall. */
result = INLINE_SYSCALL (pwrite, 6, fd, CHECK_N (buf, count), count, 0,
__LONG_LONG_PAIR (offset >> 31, offset));
# if __ASSUME_PWRITE_SYSCALL == 0
if (result == -1 && errno == ENOSYS)
/* No system call available. Use the emulation. */
result = __emulate_pwrite (fd, buf, count, offset);
# endif
return result;
}
int oldtype = LIBC_CANCEL_ASYNC ();
/* First try the syscall. */
result = INLINE_SYSCALL (pwrite, 6, fd, CHECK_N (buf, count), count, 0,
__LONG_LONG_PAIR (offset >> 31, offset));
# if __ASSUME_PWRITE_SYSCALL == 0
if (result == -1 && errno == ENOSYS)
/* No system call available. Use the emulation. */
result = __emulate_pwrite (fd, buf, count, offset);
# endif
LIBC_CANCEL_RESET (oldtype);
return result;
}
strong_alias (__libc_pwrite, __pwrite)
weak_alias (__libc_pwrite, pwrite)
# define __libc_pwrite(fd, buf, count, offset) \
static internal_function __emulate_pwrite (fd, buf, count, offset)
#endif
#if __ASSUME_PWRITE_SYSCALL == 0
# include <sysdeps/posix/pwrite.c>
#endif
Any help is appreciated.
