backend: add CardBackend::limit_card_caps

This mechanism allows the pcsc backend to signal to openpgp-card that a reader doesn't support "extended length".

card-backend/src/lib.rs

@@ -12,6 +12,11 @@
 /// A [CardBackend] is only used to get access to a [CardTransaction] object,
 /// which supports transmitting commands to the card.
 pub trait CardBackend {
+    /// If a CardBackend introduces a additional (possibly backend-specific)
+    /// limits for any fields in CardCaps, this fn can indicate that limit by
+    /// returning an amended [`CardCaps`].
+    fn limit_card_caps(&self, card_caps: CardCaps) -> CardCaps;
+
     fn transaction(
         &mut self,
         reselect_application: Option<&[u8]>,

openpgp-card/src/lib.rs

@@ -188,7 +188,7 @@ impl Card {
             let ext_cap = ard.extended_capabilities()?;
 
             // Get max command/response byte sizes from card
-            let (mut max_cmd_bytes, max_rsp_bytes) = if let Ok(Some(eli)) =
+            let (max_cmd_bytes, max_rsp_bytes) = if let Ok(Some(eli)) =
                 ard.extended_length_information()
             {
                 // In card 3.x, max lengths come from ExtendedLengthInfo
@@ -205,21 +205,6 @@ impl Card {
             let pw1_max = pw_status.pw1_max_len();
             let pw3_max = pw_status.pw3_max_len();
 
-            // If the CardTransaction implementation has an inherent limit for the cmd
-            // size, take that limit into account.
-            // (E.g. when using scdaemon as a CardTransaction backend, there is a
-            // limitation to 1000 bytes length for Assuan commands, which
-            // translates to maximum command length of a bit under 500 bytes)
-            if let Some(max_card_cmd_bytes) = tx.tx.max_cmd_len() {
-                max_cmd_bytes = u16::min(max_cmd_bytes, max_card_cmd_bytes as u16);
-            }
-
-            // FIXME: add a more general mechanism to ask the backend for
-            // amendments to the CardCaps (e.g. to change support for
-            // "extended length")
-            //
-            // Also see https://blog.apdu.fr/posts/2011/05/extended-apdu-status-per-reader/
-
             let caps = CardCaps::new(
                 ext_support,
                 chaining_support,
@@ -239,6 +224,12 @@ impl Card {
 
             drop(tx);
 
+            // General mechanism to ask the backend for amendments to
+            // the CardCaps (e.g. to change support for "extended length")
+            //
+            // Also see https://blog.apdu.fr/posts/2011/05/extended-apdu-status-per-reader/
+            let caps = op.card.limit_card_caps(caps);
+
             (caps, imm)
         };
 

pcsc/src/lib.rs

@@ -546,6 +546,25 @@ impl PcscBackend {
 }
 
 impl CardBackend for PcscBackend {
+    fn limit_card_caps(&self, card_caps: CardCaps) -> CardCaps {
+        let mut ext = card_caps.ext_support();
+
+        // Disable "extended length" support when the card reader is known not to support it
+        if self.reader_name.starts_with("ACS ACR122U") {
+            log::debug!("Disabling ext_support for reader {}", self.reader_name);
+            ext = false;
+        }
+
+        CardCaps::new(
+            ext,
+            card_caps.chaining_support(),
+            card_caps.max_cmd_bytes(),
+            card_caps.max_rsp_bytes(),
+            card_caps.pw1_max_len(),
+            card_caps.pw3_max_len(),
+        )
+    }
+
     /// Get a CardTransaction for this PcscBackend (this starts a transaction)
     fn transaction(
         &mut self,

scdc/src/lib.rs

@@ -212,6 +212,19 @@ impl ScdBackend {
 }
 
 impl CardBackend for ScdBackend {
+    fn limit_card_caps(&self, card_caps: CardCaps) -> CardCaps {
+        let max_cmd_bytes = u16::min(APDU_CMD_BYTES_MAX as u16, card_caps.max_cmd_bytes());
+
+        CardCaps::new(
+            card_caps.ext_support(),
+            card_caps.chaining_support(),
+            max_cmd_bytes,
+            card_caps.max_rsp_bytes(),
+            card_caps.pw1_max_len(),
+            card_caps.pw3_max_len(),
+        )
+    }
+
     fn transaction(
         &mut self,
         _reselect_application: Option<&[u8]>,