tar.c

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#include "c.h"
#include "pgtar.h"
#include <sys/stat.h>

/*
 * Print a numeric field in a tar header.  The field starts at *s and is of
 * length len; val is the value to be written.
 *
 * Per POSIX, the way to write a number is in octal with leading zeroes and
 * one trailing space (or NUL, but we use space) at the end of the specified
 * field width.
 *
 * However, the given value may not fit in the available space in octal form.
 * If that's true, we use the GNU extension of writing \200 followed by the
 * number in base-256 form (ie, stored in binary MSB-first).  (Note: here we
 * support only non-negative numbers, so we don't worry about the GNU rules
 * for handling negative numbers.)
 */
void
print_tar_number(char *s, int len, uint64 val)
{
    if (val < (((uint64) 1) << ((len - 1) * 3)))
    {
        /* Use octal with trailing space */
        s[--len] = ' ';
        while (len)
        {
            s[--len] = (val & 7) + '0';
            val >>= 3;
        }
    }
    else
    {
        /* Use base-256 with leading \200 */
        s[0] = '\200';
        while (len > 1)
        {
            s[--len] = (val & 255);
            val >>= 8;
        }
    }
}


/*
 * Read a numeric field in a tar header.  The field starts at *s and is of
 * length len.
 *
 * The POSIX-approved format for a number is octal, ending with a space or
 * NUL.  However, for values that don't fit, we recognize the GNU extension
 * of \200 followed by the number in base-256 form (ie, stored in binary
 * MSB-first).  (Note: here we support only non-negative numbers, so we don't
 * worry about the GNU rules for handling negative numbers.)
 */
uint64
read_tar_number(const char *s, int len)
{
    uint64      result = 0;

    if (*s == '\200')
    {
        /* base-256 */
        while (--len)
        {
            result <<= 8;
            result |= (unsigned char) (*++s);
        }
    }
    else
    {
        /* octal */
        while (len-- && *s >= '0' && *s <= '7')
        {
            result <<= 3;
            result |= (*s - '0');
            s++;
        }
    }
    return result;
}


/*
 * Calculate the tar checksum for a header. The header is assumed to always
 * be 512 bytes, per the tar standard.
 */
int
tarChecksum(char *header)
{
    int         i,
                sum;

    /*
     * Per POSIX, the checksum is the simple sum of all bytes in the header,
     * treating the bytes as unsigned, and treating the checksum field (at
     * offset 148) as though it contained 8 spaces.
     */
    sum = 8 * ' ';              /* presumed value for checksum field */
    for (i = 0; i < 512; i++)
        if (i < 148 || i >= 156)
            sum += 0xFF & header[i];
    return sum;
}


/*
 * Fill in the buffer pointed to by h with a tar format header. This buffer
 * must always have space for 512 characters, which is a requirement of
 * the tar format.
 */
enum tarError
tarCreateHeader(char *h, const char *filename, const char *linktarget,
                pgoff_t size, mode_t mode, uid_t uid, gid_t gid, time_t mtime)
{
    if (strlen(filename) > 99)
        return TAR_NAME_TOO_LONG;

    if (linktarget && strlen(linktarget) > 99)
        return TAR_SYMLINK_TOO_LONG;

    memset(h, 0, 512);          /* assume tar header size */

    /* Name 100 */
    strlcpy(&h[0], filename, 100);
    if (linktarget != NULL || S_ISDIR(mode))
    {
        /*
         * We only support symbolic links to directories, and this is
         * indicated in the tar format by adding a slash at the end of the
         * name, the same as for regular directories.
         */
        int         flen = strlen(filename);

        flen = Min(flen, 99);
        h[flen] = '/';
        h[flen + 1] = '\0';
    }

    /* Mode 8 - this doesn't include the file type bits (S_IFMT)  */
    print_tar_number(&h[100], 8, (mode & 07777));

    /* User ID 8 */
    print_tar_number(&h[108], 8, uid);

    /* Group 8 */
    print_tar_number(&h[116], 8, gid);

    /* File size 12 */
    if (linktarget != NULL || S_ISDIR(mode))
        /* Symbolic link or directory has size zero */
        print_tar_number(&h[124], 12, 0);
    else
        print_tar_number(&h[124], 12, size);

    /* Mod Time 12 */
    print_tar_number(&h[136], 12, mtime);

    /* Checksum 8 cannot be calculated until we've filled all other fields */

    if (linktarget != NULL)
    {
        /* Type - Symbolic link */
        h[156] = '2';
        /* Link Name 100 */
        strlcpy(&h[157], linktarget, 100);
    }
    else if (S_ISDIR(mode))
    {
        /* Type - directory */
        h[156] = '5';
    }
    else
    {
        /* Type - regular file */
        h[156] = '0';
    }

    /* Magic 6 */
    strcpy(&h[257], "ustar");

    /* Version 2 */
    memcpy(&h[263], "00", 2);

    /* User 32 */
    /* XXX: Do we need to care about setting correct username? */
    strlcpy(&h[265], "postgres", 32);

    /* Group 32 */
    /* XXX: Do we need to care about setting correct group name? */
    strlcpy(&h[297], "postgres", 32);

    /* Major Dev 8 */
    print_tar_number(&h[329], 8, 0);

    /* Minor Dev 8 */
    print_tar_number(&h[337], 8, 0);

    /* Prefix 155 - not used, leave as nulls */

    /* Finally, compute and insert the checksum */
    print_tar_number(&h[148], 8, tarChecksum(h));

    return TAR_OK;
}