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mbedtls: return error when retrying a blocked send with a shorter buffer

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mbedtls_ssl_write() requires the retry call after WANT_WRITE to use at
least as many bytes as the original blocked call. The DEBUGASSERT
enforcing this was compiled out in release builds, leaving the len
inflation free to exceed the caller's buffer and produce n > buflen.

Replace the assert with an explicit CURLE_SEND_ERROR so the constraint
is enforced in release builds too. Document the retry buflen requirement
in curl_easy_send(3).
This commit is contained in:
Joshua Rogers 2026-05-20 09:01:25 +02:00 • committed by Daniel Stenberg
parent f76f79696e
commit 4a999def99
2 changed files with 5 additions and 2 deletions
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4
docs/libcurl/curl_easy_send.md
View file

@ -92,7 +92,9 @@ send.
On failure, returns the appropriate error code.
This function may return **CURLE_AGAIN**. In this case, use your operating
system facilities to wait until the socket is writable, and retry.
system facilities to wait until the socket is writable, and retry. The
**buflen** passed to the retry should be no smaller than the one passed to
the call that returned **CURLE_AGAIN**.
If there is no socket available to use from the previous transfer, this
function returns **CURLE_UNSUPPORTED_PROTOCOL**.

3
lib/vtls/mbedtls.c
View file

@ -1280,7 +1280,8 @@ static CURLcode mbed_send(struct Curl_cfilter *cf, struct Curl_easy *data,
* loses bytes, e.g. reporting all was sent but they were not.
* Remember the blocked length and use that when set. */
if(backend->send_blocked) {
DEBUGASSERT(backend->send_blocked_len <= len);
if(len < backend->send_blocked_len)
return CURLE_SEND_ERROR;
CURL_TRC_CF(data, cf, "mbedtls_ssl_write(len=%zu) -> previously blocked "
"on %zu bytes", len, backend->send_blocked_len);
len = backend->send_blocked_len;