Date: Mon, 21 Feb 2011 17:06:18 +0100 From: Mark Martinec <Mark.Martinec+freebsd@ijs.si> To: freebsd-fs@freebsd.org Subject: ftruncate under ZFS requires W file access permission, instead of testing file open mode (O_RDWR) Message-ID: <201102211706.18084.Mark.Martinec%2Bfreebsd@ijs.si>
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I filed this as: http://www.freebsd.org/cgi/query-pr.cgi?pr=154873 under a 'standards' category - should be assigned to [freebsd-fs]: standards/154873: ZFS violates POSIX on open/O_CREAT -> ftruncate Description: POSIX.1-2008 requires that the third argument to open(2) on O_CREAT does not affect whether the file is open for reading, writing, or for both. A subsequent ftruncate should not depend on access permission bits of the file, but solely on the read/write flags specified on the open(2). The bug affects a mailer Postfix, which reports a permission problem on ftruncate, affecting the smtpd service when option speed_adjust is requested. As the problem is in the file system violating a POSIX specification and not in the application, it is unlikely that the program will be modified. Reproducible on: FreeBSD 8.1-RELEASE-p2, ZFS: pool version 14, ZFS version 3 as well as on: FreeBSD 8.2-RC3, ZFS: pool version 15, ZFS version 4 How-To-Repeat: Run the following test program. It will report: Error truncating: Permission denied when (cwd) on a ZFS file system, but will pass clean when on UFS or on some other file system. #include <stdio.h> #include <unistd.h> #include <fcntl.h> int main(int argc, char *argv[]) { const char fname[] = "truncate-posix-test.tmp"; /* POSIX.1-2008: ( http://www.opengroup.org/onlinepubs/9699919799/ ) * open() [...] O_CREAT [...] The file status flags and file * access modes of the open file description shall be set according * to the value of oflag. [...] The argument following the oflag * argument does not affect whether the file is open for reading, * writing, or for both. * * In other words, read/write access is controlled with the * O_RDWR flags, not the read/write permissions argument. * * Create a file with mode 0, i.e. all access permission bits off: */ int fd = open(fname, O_CREAT|O_RDWR|O_EXCL, 0); if (fd < 0) { perror("Error creating file"); return 1; } if (unlink(fname) < 0) perror("Error unlinking"); /* ftruncate should succeed, * it must not depend on access permission bits, its rights * should solely be governed by an O_RDWR file access flag. * * This FAILS on a ZFS file system, reporting "Permission denied"! */ if (ftruncate(fd,0) < 0) perror("Error truncating"); if (close(fd) < 0) perror("Error closing"); return 0; } Fix: Fix unknown. Work around by not using ZFS, or by allowing less strict access permission bits on open(2) - which may be hard to achieve if code is buried inside some application. Mark
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