If the account length is zero, no data region is present (#5122)

This only affects unaligned programs, as for aligned programs the resize
area address will match the account region address if the data length is
zero.

programs/bpf_loader/src/syscalls/mem_ops.rs

@@ -530,7 +530,7 @@ impl<'a> Iterator for MemoryChunkIterator<'a> {
                     account_index = account_index.saturating_add(1);
                     self.account_index = Some(account_index);
                 } else {
-                    region_is_account = region.vm_addr == account_addr
+                    region_is_account = (account.original_data_len != 0 && region.vm_addr == account_addr)
                         // unaligned programs do not have a resize area
                         || (self.resize_area && region.vm_addr == resize_addr);
                     break;
@@ -604,7 +604,7 @@ impl DoubleEndedIterator for MemoryChunkIterator<'_> {
 
                 self.account_index = Some(account_index);
             } else {
-                region_is_account = region.vm_addr == account_addr
+                region_is_account = (account.original_data_len != 0 && region.vm_addr == account_addr)
                     // unaligned programs do not have a resize area
                     || (self.resize_area && region.vm_addr == resize_addr);
                 break;

programs/sbf/rust/account_mem/src/lib.rs

@@ -60,8 +60,15 @@ pub fn process_instruction(
             sol_memset(&mut data[data_len.saturating_sub(2)..], 0, 3);
         }
         5 => {
+            // memcmp overlaps begining
+            #[allow(clippy::manual_memcpy)]
+            for i in 0..3 {
+                buf[i] = too_early(2)[i];
+            }
+
             // memset overlaps begin of account area
             sol_memset(too_early(2), 3, 3);
+            sol_memcpy(too_early(2), &buf, 3);
         }
         6 => {
             // memcpy src overlaps end of account

programs/sbf/rust/account_mem_deprecated/src/lib.rs

@@ -25,7 +25,7 @@ pub fn process_instruction(
     let mut too_early = |before: usize| -> &mut [u8] {
         let data = data.as_mut_ptr().wrapping_sub(before);
 
-        unsafe { std::slice::from_raw_parts_mut(data, data_len) }
+        unsafe { std::slice::from_raw_parts_mut(data, data_len.wrapping_add(100)) }
     };
 
     match instruction_data[0] {
@@ -40,11 +40,11 @@ pub fn process_instruction(
         2 => {
             // memcmp overlaps begining
             #[allow(clippy::manual_memcpy)]
-            for i in 0..500 {
+            for i in 0..90 {
                 buf[i] = too_early(8)[i];
             }
 
-            sol_memcmp(too_early(8), &buf, 500);
+            sol_memcmp(too_early(8), &buf, 90);
         }
         3 => {
             // memcmp overlaps begining

programs/sbf/tests/programs.rs

@@ -5194,6 +5194,29 @@ fn test_mem_syscalls_overlap_account_begin_or_end() {
                     assert!(!logs.last().unwrap().ends_with(" failed: InvalidLength"));
                 }
             }
+
+            let account = AccountSharedData::new(42, 0, &program_id);
+            bank.store_account(&account_keypair.pubkey(), &account);
+
+            for instr in 0..=15 {
+                println!("Testing deprecated:{deprecated} direct_mapping:{direct_mapping} instruction:{instr} zero-length account");
+                let instruction =
+                    Instruction::new_with_bytes(program_id, &[instr, 0], account_metas.clone());
+
+                let message = Message::new(&[instruction], Some(&mint_pubkey));
+                let tx = Transaction::new(&[&mint_keypair], message.clone(), bank.last_blockhash());
+                let (result, _, logs, _) = process_transaction_and_record_inner(&bank, tx);
+
+                if direct_mapping && !deprecated {
+                    // we have a resize area
+                    assert!(
+                        logs.last().unwrap().ends_with(" failed: InvalidLength"),
+                        "{logs:?}"
+                    );
+                } else {
+                    assert!(result.is_ok(), "{logs:?}");
+                }
+            }
         }
     }
 }