Re: Bug#659499: bash fails to properly read /proc files

[Cyril Brulebois <kibi@debian.org>]

Hi kernel folks,

here's a tiny analysis I tried to perform on bash's having issues with
reading /proc files, which I think is related to seeking in those files.
I can't play much with other kernel versions right now though. My tests
were performed with squeeze's bpo kernel: 3.2.0-0.bpo.1-amd64 (Debian
3.2.4-1~bpo60+1).

I'm stripping parts of my explanations of why other shells seem
unaffected (different implementations for reading files).

Cyril Brulebois <kibi@debian.org> (18/02/2012):
> strace shows it's reading 128 bytes, then trying to adjust using
> lseek(), which might explain what you're seeing if the read() + lseek()
> calls trigger nasty things.

Attached is a reduced (as in “lighter than bash”) test case. The code is
ugly but I'm throwing it over the wall before the BSP's end: built with
bufsize=8000, everything is fine for my 600-ish bytes /proc/net/dev;
built with bufsize=128, read()+lseek() seem to trigger nasty stuff as I
suspected.

Here's the output for bufsize=8000:
| $ gcc mini-test.c && ./a.out 
| Warning: no file specified, defaulting to /proc/net/dev
| Info: /proc/net/dev opened successfully
| Read: 694
| Found newline: 76 char-long line: with 617 extra chars:
| Inter-|   Receive                                                |  Transmit
| Read: 617
| Found newline: 122 char-long line: with 494 extra chars:
|  face |bytes    packets errs drop fifo frame compressed multicast|bytes    packets errs drop fifo colls carrier compressed
| Read: 494
| Found newline: 122 char-long line: with 371 extra chars:
|     lo:   63886     451    0    0    0     0          0         0    63886     451    0    0    0     0       0          0
| Read: 371
| Found newline: 122 char-long line: with 248 extra chars:
|   pan0:       0       0    0    0    0     0          0         0        0       0    0    0    0     0       0          0
| Read: 248
| Found newline: 124 char-long line: with 123 extra chars:
|  wlan0: 151354717  197302    0    0    0     0          0         0 22011993  189809    0    0    0     0       0          0
| Read: 123
| Found newline: 122 char-long line: with 0 extra chars:
|   eth0:       0       0    0    0    0     0          0         0        0       0    0    0    0     0       0          0
| Final position: 694
→ OK

Here's the output for bufsize=128:
| $ gcc mini-test.c && ./a.out 
| Warning: no file specified, defaulting to /proc/net/dev
| Info: /proc/net/dev opened successfully
| Read: 128
| Found newline: 76 char-long line: with 51 extra chars:
| Inter-|   Receive                                                |  Transmit
| Read: 128
| Found newline: 122 char-long line: with 5 extra chars:
|  face |bytes    packets errs drop fifo frame compressed multicast|bytes    packets errs drop fifo colls carrier compressed
| Read: 128
| Found newline: 122 char-long line: with 5 extra chars:
|   pan0:       0       0    0    0    0     0          0         0        0       0    0    0    0     0       0          0
| Final position: 323

Has anything like that been reported/fixed recently?


(Probably my last one:) Thanks to IRILL for sponsoring this BSP in Paris.

Mraw,
KiBi.

#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
#include <string.h>

const char default_input[] = "/proc/net/dev";

int main(int argc, const char **argv) {
  const char *input = default_input;

  /* "usage" */
  if (argc < 2) {
    printf("Warning: no file specified, defaulting to %s\n", default_input);
  } else {
    input = argv[1];
    printf("Info: using %s as specified\n", argv[1]);
  }
  
  int file = open(input, 0);
  if (file < 0) {
    printf("Error: unable to open %s: %s\n", input, strerror(errno));
    return 1;
  } else {
    ssize_t ret;
    ssize_t current = 0;
    ssize_t extra = 0;

    printf("Info: %s opened successfully\n", input);

    /* this is ugly but oh well */
    #define bufsize (8000)

    char buf[bufsize+1] = "";
    while ((ret = read(file, buf, bufsize)) > 0) {
      printf("Read: %u\n", ret);
      char *newline = strchr(buf, '\n');
      if (newline) {
        current += (newline-buf);
        *newline = '\0';
        extra = ret-(newline-buf+1);
        printf("Found newline: %u char-long line: with %u extra chars:\n%s\n", current, extra, buf);
        lseek(file, -extra, SEEK_CUR);
        current = 0;
      }
    }
    printf("Final position: %u\n", lseek(file, 0, SEEK_CUR)); 
    close(file);
  }

  return 0;
}

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