Reg: Freeing of SSL_CTX object through SSL_free Function
matt at openssl.org
Wed Apr 13 11:28:45 UTC 2022
On 13/04/2022 11:55, Ram Chandra via openssl-users wrote:
> I am using OpenSSL 1.1.1k Version, From that I see following are done in
> OpenSSL code, pls correct me if I am wrong.
> Inside SSL_new:
> SSL *SSL_new(SSL_CTX *ctx)
> SSL *s;
> s = OPENSSL_zalloc(sizeof(*s));
> s->ctx = ctx; /* *ctx* value to *s->ctx* , also gets freed when
> called SLL_free(s) ==> SSL_CTX_free(s->ctx)*/
> s->session_ctx = ctx;/* same value is getting assigned here also */
> return s;
> Inside SSL_free:
> void SSL_free(SSL *s)
> if ( s == NULL)
> SSL_CTX_free( s->session_ctx); /* this holds the value of ctx that
> was passed t
> o SSL_new(), yes or no? */
> SSL_CTX_free( s->ctx); /* this again trying to free the same
> pointer , abnormal behavior */
> Point here is inside SSL_CTX_free(), after freeing "s->session_ctx" ,
> we are not setting "s->session_ctx" to NULL(this may be optional, its ok
> if we don't use the same pointer again), but "s->session_ctx"
> and "s->ctx" both have same value. So applying "free()" on same value
> again ( through SSL_CTX_free( s->ctx); ) will result in abnormal
> behavior, *correct or not?*
Not correct. SSL_CTX_free() may not actually free the object at all -
> I could not understand how OpenSSL *free() *ing pointers if they are
> assigned to multiple different variables.
> Note: tried going through "SSL_CTX_up_ref(ctx);" , and
> "SSL_CTX_down_ref(ctx);", looks like they are tracking the pointer usage
> count by other APIs, but could not understand what exactly they are
> doing...when count is 0.
SSL_CTX objects are reference counted. So SSL_CTX_up_ref() increments
the reference count and SSL_CTX_free() decrements it. A reference is
created for each place that you assign an object to a different
location. Only when all references are freed (i.e. when the reference
count drops to 0) does the object itself actually get freed.
> Could someone please elaborate a bit ..
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