Re: utmpx support

[finkler <finkler@officinamentis.org>]

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On 06.03.2012 02:30, Rich Felker wrote:
> On Sun, Mar 04, 2012 at 08:10:36PM +0100, finkler wrote:
> Append mode is definitely not correct if you want to be able to
> overwrite existing entries.
Changed it to r+ and r

> I'm confused about the intent of this code.. Presumably it's supposed
> to modify the entry it just found (relying on getutxid leaving the
> file at the right position), but that's not going to work because the
> file is opened in append mode.
>
> Also, -(sizeof ut) is not a valid offset; it's almost SIZE_MAX. If you
> were using fseeko, this would result in a gigantic offset (rather than
> a negative one) after the conversion; with fseek (which takes a long),
> it should convert back to the desired negative value, but for safety
> you should cast to a signed type *before* negating the result of
> sizeof
Made the cast inside the function call.

> There's also the issue that there's absolutely no locking on updates,
> so if multiple apps try to add/update utmp entries at the same time,
> the file would be corrupted. This could be fixed by using an fcntl
> lock on fileno(f) for the duration of the function.
I added a lock, however, I am unsure whether I should use a lock that 
waits until it is free or not, and whether I should lock the whole file, 
or just the portion which is about to be written.
Right now I use a waiting lock, and lock the whole file.

 > Here strcmp is being called on data that was read from a file with no
 > validation. This is potentially a security issue (DoS); if the file
 > does not contain a null-terminated string, strcmp could run past the
 > end of the buffer and eventually segfault and crash the calling
 > program. It's probably hard to trigger the issue since the string for
 > comparison is also located in a utmp structure, but I think there
 > should be some validation, probably just fixing invalid data right
 > after the fread call so it never leaks out.
I changed it to use strncmp with the sizeof the respective struct field, 
is this enough?

Thank you very much for your time, I attached the reworked version.

[-- Attachment #2: utmpx.c --]
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#include <utmpx.h>
#include <stddef.h>
#include "libc.h"

#define _PATH_UTMPX "/etc/utmp"

static FILE *f;
static struct utmpx ut;

void endutxent(void)
{
  if (f == NULL) return;
  fclose(f);
  f == NULL;
}

void setutxent(void)
{
  if (f == NULL) return;
  rewind(f);
}

struct utmpx *getutxent(void)
{
  if (f == NULL) {
    f = fopen(_PATH_UTMPX, "r+");
    if (f == NULL) {
      f = fopen(_PATH_UTMPX, "r");
      if (f == NULL) return NULL;
    }
  }
  if (fread(&ut, sizeof ut, 1, f)) return &ut;
  return NULL;
}

struct utmpx *getutxid(const struct utmpx *id)
{  
  while(getutxent()) {
    switch (id->ut_type) {
      case BOOT_TIME:
      case OLD_TIME:
      case NEW_TIME:
        if (id->ut_type == ut.ut_type) return &ut;
        break;
      case INIT_PROCESS:
      case LOGIN_PROCESS:
      case USER_PROCESS:
      case DEAD_PROCESS:
        if (id->type == ut.ut_type && !strncmp(id->ut_id, ut.ut_id, sizeof ut.ut_id)) return &ut;
        break;
    }
  }    
  return NULL;
}

struct utmpx *getutxline(const struct utmpx *line)
{
	while(getutxent()) {
    switch (ut.ut_type) {
      case LOGIN_PROCESS:
      case USER_PROCESS:
        if (!strncmp(line->ut_line, ut.ut_line, sizeof ut.ut_line)) return &ut;
        break;
    }
  }
  return NULL;
}

struct utmpx *pututxline(const struct utmpx *utmpx)
{
  struct flock fl;
  size_t n;
  
  fl.l_start = 0;
  fl.l_len = 0;
  fl.l_pid = getpid();
  fl.l_type = F_WRLCK;
  fl.l_whence = SEEK_SET;
  
  if (fcntl(fileno(f), F_SETLKW, &fl) < 0) return NULL;
  
  setutxent();
  
  if (getutxid(utmpx)) fseek(f, -((long)(sizeof ut)), SEEK_CUR);
  
  n = fwrite(&ut, sizeof ut, 1, f);
  
  fl.l_type = F_UNLCK;
  fcntl(fileno(f), F_SETLK, &fl);
  
  if (n == 1)
    return &ut;
  return NULL;
}

void updwtmpx(const char *f, const struct utmpx *u)
{
}

weak_alias(endutxent, endutent);
weak_alias(setutxent, setutent);
weak_alias(getutxent, getutent);
weak_alias(getutxid, getutid);
weak_alias(getutxline, getutline);
weak_alias(pututxline, pututline);
weak_alias(updwtmpx, updwtmp);