On Sun, Jan 31, 2021 at 05:58:39AM +0700, Eugene Grosbein wrote:
> 31.01.2021 5:26, Jilles Tjoelker wrote:
> >>> +static bool
> >>> +isbinary(const char *data, size_t len)
> >>> +{
> >>> + const char *nul, *p;
> >>> + bool hasletter;
> >>> +
> >>> + nul = memchr(data, '\0', len);
> >>> + if (nul == NULL)
> >>> + return false;
> >>> + /*
> >>> + * POSIX says we shall allow execution if the initial part intended
> >>> + * to be parsed by the shell consists of characters and does not
> >>> + * contain the NUL character. This allows concatenating a shell
> >>> + * script (ending with exec or exit) and a binary payload.
> >>> + *
> >>> + * In order to reject common binary files such as PNG images, check
> >>> + * that there is a lowercase letter or expansion before the last
> >>> + * newline before the NUL character, in addition to the check for
> >>> + * the newline character suggested by POSIX.
> >>> + */
> >>> + hasletter = false;
> >>> + for (p = data; *p != '\0'; p++) {
> >>> + if ((*p >= 'a' && *p <= 'z') || *p == '$' || *p == '`')
> >>> + hasletter = true;
> >>> + if (hasletter && *p == '\n')
> >>> + return false;
> >>> + }
> >>> + return true;
> >>> +}
> >> Before last newline or before first newline?
> > Before the last newline, according to both comment and code.
> Sorry, I don't get it. The "for" loop starts from the beginning, and
> returns false (NOT binary, text file) after lowercase letter and first
> newline, not last.
The loop continues until the first NUL byte and will return false (not
binary) when it encounters any newline after an ASCII lowercase letter,
'$' or '`'.
--
Jilles Tjoelker
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