Fix incorrect nonoverlapping test in sol_memcpy (backport #21007) (#23512)

* Fix incorrect nonoverlapping test in sol_memcpy (#21007)

Thanks!

(cherry picked from commit df2b448993)

# Conflicts:
#	programs/bpf_loader/src/syscalls.rs
#	sdk/program/src/program_stubs.rs
#	sdk/src/feature_set.rs

* resolve conflicts

Co-authored-by: Brian Anderson <andersrb@gmail.com>
Co-authored-by: Jack May <jack@solana.com>

sdk/program/src/program_stubs.rs

@@ -54,7 +54,7 @@ pub trait SyscallStubs: Sync + Send {
     unsafe fn sol_memcpy(&self, dst: *mut u8, src: *const u8, n: usize) {
         // cannot be overlapping
         assert!(
-            !(dst as usize + n > src as usize && src as usize > dst as usize),
+            is_nonoverlapping(src as usize, dst as usize, n),
             "memcpy does not support overlapping regions"
         );
         std::ptr::copy_nonoverlapping(src, dst, n as usize);
@@ -193,3 +193,35 @@ pub(crate) fn sol_get_processed_sibling_instruction(index: usize) -> Option<Inst
 pub(crate) fn sol_get_stack_height() -> u64 {
     SYSCALL_STUBS.read().unwrap().sol_get_stack_height()
 }
+
+/// Check that two regions do not overlap.
+///
+/// Adapted from libcore, hidden to share with bpf_loader without being part of
+/// the API surface.
+#[doc(hidden)]
+pub fn is_nonoverlapping<N>(src: N, dst: N, count: N) -> bool
+where
+    N: Ord + std::ops::Sub<Output = N>,
+    <N as std::ops::Sub>::Output: Ord,
+{
+    let diff = if src > dst { src - dst } else { dst - src };
+    // If the absolute distance between the ptrs is at least as big as the size of the buffer,
+    // they do not overlap.
+    diff >= count
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_is_nonoverlapping() {
+        assert!(is_nonoverlapping(10, 7, 3));
+        assert!(!is_nonoverlapping(10, 8, 3));
+        assert!(!is_nonoverlapping(10, 9, 3));
+        assert!(!is_nonoverlapping(10, 10, 3));
+        assert!(!is_nonoverlapping(10, 11, 3));
+        assert!(!is_nonoverlapping(10, 12, 3));
+        assert!(is_nonoverlapping(10, 13, 3));
+    }
+}

sdk/src/feature_set.rs

@@ -303,6 +303,10 @@ pub mod add_get_processed_sibling_instruction_syscall {
     solana_sdk::declare_id!("CFK1hRCNy8JJuAAY8Pb2GjLFNdCThS2qwZNe3izzBMgn");
 }
 
+pub mod fixed_memcpy_nonoverlapping_check {
+    solana_sdk::declare_id!("36PRUK2Dz6HWYdG9SpjeAsF5F3KxnFCakA2BZMbtMhSb");
+}
+
 lazy_static! {
     /// Map of feature identifiers to user-visible description
     pub static ref FEATURE_NAMES: HashMap<Pubkey, &'static str> = [
@@ -373,6 +377,7 @@ lazy_static! {
         (disable_bpf_deprecated_load_instructions::id(), "Disable ldabs* and ldind* BPF instructions"),
         (disable_bpf_unresolved_symbols_at_runtime::id(), "Disable reporting of unresolved BPF symbols at runtime"),
         (add_get_processed_sibling_instruction_syscall::id(), "add add_get_processed_sibling_instruction_syscall"),
+        (fixed_memcpy_nonoverlapping_check::id(), "use correct check for nonoverlapping regions in memcpy syscall"),
         /*************** ADD NEW FEATURES HERE ***************/
     ]
     .iter()