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 <k...@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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