Re: [PATCH] help: Fix size passed to qsort

From: Junio C Hamano <gitster@pobox.com>
Date: Sep 18, 2014, 17:34 UTC
Message-ID: <xmqqzjdwomrr.fsf@gitster.dls.corp.google.com>
In-Reply-To: <xmqq8ulgq25h.fsf@gitster.dls.corp.google.com>

Junio C Hamano <gitster@pobox.com> writes:

Show 19 quoted lines

> Stefan Beller <stefanbeller@gmail.com> writes:
>
>> We actually want to have the size of one 'name' and not the size
>> of the names array.
> ...
> I suspect that the latter is "size of a pointer that points at a
> cmdname structure", but the original code in help_unknown_cmd() is
> wrong.  The ones in load_command_list() do this correctly and
> another qsort() invocation in this function does so as well.  I
> wonder why they didn't correctly cut&paste ;-)
>
> 746c221a (git wrapper: also use aliases to correct mistyped
> commands, 2008-09-10) seemed to have introduced the culprit.
>
> The call to uniq() would fail to uniquify because main_cmds would
> have the standard command all in front and then aliases and commands
> in the user's PATH later, but I do not quite see if there is any
> end-user observable breakages that can arise from this.  What is the
> practical implication of this breakage?

Heh, I should have spent a bit more time before starting to type. The answer probably is "nothing", as

	struct cmdnames {
        	...
                struct cmdname {
                	...
		} **names;
	} main_cmds;

is what we are dealing here, so main_cmds.names is a pointer that points at a slab of memory to hold many pointers, each of which points at "struct cmdname". And sizeof(struct cmdname **) is incorrectly passed where we should pass sizeof(struct cmdname *), which you fixed, but in practice they would be the same size anyway ;-)

Show 6 quoted lines

>>  	qsort(main_cmds.names, main_cmds.cnt,
>> -	      sizeof(main_cmds.names), cmdname_compare);
>> +	      sizeof(*main_cmds.names), cmdname_compare);
>>  	uniq(&main_cmds);
>>  
>>  	/* This abuses cmdname->len for levenshtein distance */