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Code Block

bgColor	#FFCCCC

char *file namefilename;
struct stat lstat_info;
int fd;
/* ... */
if (lstat(file namefilename, &lstat_info) == -1) {
  /* handle error */
}
if (!S_ISLNK(lstat_info.st_mode)) {
   if ((fd = open(file namefilename, O_RDWR)) == -1) {
       /* handle error */
   }
}
write(fd, userbuf, userlen);

...

This compliant solution eliminates the race condition by:

calling lstat() on the file namefilename.
calling open() to open the file.
calling fstat() on the file descriptor returned by open().
comparing the file information returned by the calls to lstat() and fstat() to ensure that the files are the same.

Code Block

bgColor	#ccccff

char *file namefilename;
struct stat lstat_info
struct stat fstat_info;
int fd;
/* ... */
if (lstat(file namefilename, &lstat_info) == -1) {
  /* handle error */
}
if ((fd = open(file namefilename, O_RDWR)) == -1) {
  /* handle error */
}
if (fstat(fd, &fstat_info) == -1) {
  /* handle error */
}
if (lstat_info.st_mode == fstat_info.st_mode &&
    lstat_info.st_ino == fstat_info.st_ino  &&
    lstat_info.st_dev == fstat_info.st_dev) {
  write(fd, userbuf, userlen);
}

...

Wiki Markup

\[[Dowd 06|AA. C References#Dowd 06]\] Chapter 9, "UNIX 1: Privileges and Files"
\[[ISO/IEC 9899:-1999|AA. C References#ISO/IEC 9899-1999]\] Section 7.19, "Input/output <stdio.h>"
\[[MITRE 07|AA. C References#MITRE 07]\] [CWE ID 365|http://cwe.mitre.org/data/definitions/365.html], "Race Condition in Switch"
\[[Open Group 04|AA. C References#Open Group 04]\] [lstat()|http://www.opengroup.org/onlinepubs/000095399/functions/lstat.html], [fstat()|http://www.opengroup.org/onlinepubs/009695399/functions/fstat.html], [open()|http://www.opengroup.org/onlinepubs/009695399/functions/open.html]
\[[Seacord 05|AA. C References#Seacord 05]\] Chapter 7, "File I/O"

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