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From: Alexander Monakov <amonakov@ispras.ru>
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Subject: Re: [PATCH] fix use of pointer after free in unsetenv
Date: Mon, 4 Jan 2016 18:47:36 +0300 (MSK)
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On Mon, 4 Jan 2016, Alexander Monakov wrote:
> To me the implementation looks weird due to how it restarts scanning __environ
> with 'goto again' from position 0 instead of current position. I can propose
> the following rewrite (untested):
> 
> for (i=0; __environ[i]; i++) {
> 	char *e = __environ[i];
> 	if (!memcmp(name, e, l) && e[l] == '=') {
> 		for (j=i--; __environ[j]; j++)
> 			__environ[j] = __environ[j+1];
> 		if (__env_map) {
> 			for (j=0; __env_map[j] && __env_map[j] != e; j++);
> 			if (__env_map[j]) {
> 				free(__env_map[j]);
> 				do __env_map[j] = __env_map[j+1];
> 				while (__env_map[++j]);
> 			}
> 		}
> 	}
> }

Hm, there's no need to preserve relative order of env entries, is there?

for (i=0; __environ[i]; i++);
for (int im=i-1; i-->0; )
	if (!memcmp(name, __environ[i], l) && __environ[i][l] == '=') {
		if (__env_map) {
			for (j=0; __env_map[j]; j++);
			for (int jm=j-1; j-->0; )
				if (__env_map[j] == __environ[i]) {
					__env_map[j] = __env_map[jm];
					__env_map[jm] = 0;
					free(__environ[i]);
					break;
				}
		}
		if (i != im)
			__environ[i] = __environ[im];
		__environ[im--] = 0;
	}

(in practice I'd rather spell i-->0 as --i>=0 above)

Alexander