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Re: libcurl documentation bug

From: Short, Todd <>
Date: Fri, 16 Sep 2016 12:44:20 +0000

The scenario I’m thinking of is:

1. initialize OpenSSL
2. use OpenSSL ex_data
3. curl_global_init();
4. use curl
5. curl_global_cleanup(); /* which calls CRYPTO_cleanup_all_ex_data() */
6. continue to use OpenSSL ex_data — error/crash

> On Sep 16, 2016, at 3:58 AM, John Marshall <> wrote:
> On 15 Sep 2016, at 14:53, Short, Todd <> wrote:
>> We’ve discovered a bug in the libcurl documentation. The issue revolves around curl_global_cleanup() that ends up calling CRYPTO_cleanup_all_ex_data(). This OpenSSL function is meant to be called only once, at program termination, as it cleans up data that leaves users of ex_data in an inconsistent state. (tl;dr: ex_data index values are basically reset, but are still held by application).
>> At referring to curl_global_init() and curl_global_cleanup():
>> “You can call both of these multiple times, as long as all calls meet these requirements and the number of calls to each is the same.”
>> This statement is wrong, as curl_global_cleanup() cannot be called multiple times due to CRYPTO_cleanup_all_ex_data().
> Perhaps I am missing something here, but the curl_global_* functions maintain a counter so if the calls are nested, e.g. curl_global_init / curl_global_init / curl_global_cleanup / curl_global_cleanup, then only the last call to curl_global_cleanup() actually calls CRYPTO_cleanup_all_ex_data(). So all is well, no?
> Or are you thinking of a scenario like curl_global_init / curl_global_cleanup ... curl_global_init / curl_global_cleanup in which curl gets torn down and later re-inited? And you're saying that Curl_ossl_init() (called by curl_global_init()) does not and/or cannot sufficiently reinitialise this data?

The issue is that curl_global_cleanup() ends up calling CRYPTO_cleanup_all_ex_data() which frees OpenSSL data structures that effectively leave dangling indexes that don’t get cleaned up, as there is no mechanism to do so, because CRYPTO_cleanup_all_ex_data() must only be called after all OpenSSL use is complete.

> John
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-Todd Short
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Received on 2016-09-16