From owner-freebsd-hackers  Tue Aug 24 13:28:21 1999
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From: "John W. DeBoskey" <jwd@unx.sas.com>
Message-Id: <199908242025.QAA24724@bb01f39.unx.sas.com>
Subject: seek to negative offset?
To: freebsd-hackers@freebsd.org
Date: Tue, 24 Aug 1999 16:25:26 -0400 (EDT)
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Hi,

   The subject says it all... We have some code that scans files
backwards... On the systems we've been running this program on, the
lseek man page typically lists:

      [EINVAL]       The resulting file offset would be negative.


   The following program run under freebsd-current shows the problem:

#include <stdio.h>
#include <errno.h>
#include <unistd.h>
#include <fcntl.h>

main() 
{
   int offset, myerrno;
   int myfd;
   
   myfd = open("/etc/passwd", O_RDONLY);

   printf("Seeking to byte 512...\n");
   offset = lseek(myfd, 512, SEEK_SET);
   myerrno = errno;
   printf("offset after first lseek: %d\nerrno=%d\n\n",offset,myerrno);

   printf("Seeking relative -1024 bytes...\n");
   offset = lseek(myfd, -1024, SEEK_CUR);
   myerrno = errno;
   printf("offset after second lseek: %d\nerrno=%d\n\n",offset,myerrno);

   printf("Current offset location: %d\n",lseek(myfd, 0, SEEK_CUR));

   close(myfd);

}

sample output:

Seeking to byte 512...
offset after first lseek: 512
errno=0

Seeking relative -1024 bytes...
offset after second lseek: -512
errno=0

Current offset location: -512



   In looking through /usr/src/sys/kern/vfs_syscalls.c I can't see
where we do any validation on the resulting seek location... Do the
appropriate folks think this is a bug? How about posix? Should I 
go ahead and submit a pr with a patch?

Thanks!
John


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