git: 3708b615c354 - stable/12 - sh: Allow more scripts without #!

[Jilles Tjoelker]

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