[bug report] fs: fat: create correct short names

[Dan Carpenter]

Hello Heinrich Schuchardt,

The patch 28cef9ca2e86: "fs: fat: create correct short names" from
Nov 20, 2020, leads to the following Smatch static
checker warning:

    fs/fat/fat_write.c:61 str2fat()
    warn: impossible condition '(c > 127) => ((-128)-127 > 127)'

    fs/fat/fat_write.c:136 set_name()
    warn: impossible condition '(*dirent.name == 229) => ((-128)-127 == 229)'

fs/fat/fat_write.c
    105 static int set_name(fat_itr *itr, const char *filename, char *shortname)
    106 {
    107         char *period;
    108         char *pos;
    109         int period_location;
    110         char buf[13];
    111         int i;
    112         int ret;
    113         struct nameext dirent;
    114 
    115         if (!filename)
    116                 return -EIO;
    117 
    118         /* Initialize buffer */
    119         memset(&dirent, ' ', sizeof(dirent));
    120 
    121         /* Convert filename to upper case short name */
    122         period = strrchr(filename, '.');
    123         pos = (char *)filename;
    124         if (*pos == '.') {
    125                 pos = period + 1;
    126                 period = 0;
    127         }
    128         if (period)
    129                 str2fat(dirent.ext, period + 1, sizeof(dirent.ext));
    130         period_location = str2fat(dirent.name, pos, sizeof(dirent.name));
    131         if (period_location < 0)
    132                 return period_location;
    133         if (*dirent.name == ' ')
    134                 *dirent.name = '_';
    135         /* 0xe5 signals a deleted directory entry. Replace it by 0x05. */
--> 136         if (*dirent.name == 0xe5)

dirent.name is a char type.  Chars are signed on probably most people's
systems.  The Linux kernel recently made char unsigned for everyone.
Anyway in a char 0xe5 is negative but as a literal, it's positive so
this condition cannot be true.

    137                 *dirent.name = 0x05;
    138 
    139         /* If filename and short name are the same, quit. */
    140         sprintf(buf, "%.*s.%.3s", period_location, dirent.name, dirent.ext);
    141         if (!strcmp(buf, filename)) {
    142                 ret = 1;

regards,
dan carpenter