Re: readlink() example sometimes fails

["Michael Kerrisk (man-pages)" <mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

Hello Vladimir,

On 08/18/2016 05:30 PM, Ursache Vladimir wrote:
>  Hi,
> the readlink() example from:
> 
> http://man7.org/linux/man-pages/man2/readlink.2.html
> 
> relies on lstat()'s st_size, but that doesn't work for files contents
> created dynamically by the Linux kernel which often report a zero
> size, for example the link at:
> /sys/block/sda
> 
> the example code will fail because stat.st_size reports zero and you
> try to read (stat.st_size + 1) which will succeed, which will generate
> the error : "symlink increased in size between lstat() and
> readlink()".
> 
> Somewhat related, the same issue is true for reading regular text
> files, e.g: "/proc/filesystems" which will report stat.st_size = 0.
> 
> My quick workaround:
> 
> if (stat.st_size != 0)
>     // work as usual
> else if (file_is_a_link)
>     // malloc 4K of ram and try to readlink() into it
> else if (is_regular_file)
>     // read() into a byte array that grows accordingly
> 
> In case it matters, I'm using Ubuntu 16.04 amd64.

Thanks for this report. I agree that the page needs to be fixed.
I modified the example program to be as follows:

#include <sys/types.h>
#include <sys/stat.h>
#include <limits.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int
main(int argc, char *argv[])
{
    struct stat sb;
    char *linkname;
    ssize_t r, bufsiz;

    if (argc != 2) {
        fprintf(stderr, "Usage: %s <pathname>\n", argv[0]);
        exit(EXIT_FAILURE);
    }

    if (lstat(argv[1], &sb) == -1) {
        perror("lstat");
        exit(EXIT_FAILURE);
    }

    bufsiz = sb.st_size + 1;

    /* Some magic symlinks under (for example) /proc and /sys
       report 'st_size' as zero. In that case, take PATH_MAX as
       a "good enough" estimate */

    if (sb.st_size == 0)
	bufsiz = PATH_MAX;

    printf("%zd\n", bufsiz);

    linkname = malloc(bufsiz);
    if (linkname == NULL) {
        perror("malloc");
        exit(EXIT_FAILURE);
    }

    r = readlink(argv[1], linkname, bufsiz);
    if (r == -1) {
        perror("readlink");
        exit(EXIT_FAILURE);
    }

    linkname[r] = '\0';

    printf("'%s' points to '%s'\n", argv[1], linkname);

    if (r == bufsiz)
        printf("(Returned buffer may have been truncated)\n");

    free(linkname);
    exit(EXIT_SUCCESS);
}

Seem okay?

Cheers,

Michael


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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