short: shorten the shorter prestub

Author: byeongkeunahn

scripts/static-pie-prestub-amd64-shorter.asm

@@ -64,4 +64,4 @@ _decode_zeros:
 ; Jump to entrypoint
 _jump_to_entrypoint:
     sub     rdi, qword [rdi-8]
-    jmp     rdi
+    call    rdi

scripts/static-pie-prestub-amd64-shorter.bin

@@ -1 +1 @@
-j	X�E1�V1�j
^�j"AZj�AX^H�����,#�r#t�Ȓ�,$k�[
����u����O�H�����H+���
+j	X�E1�V1�j
^�j"AZj�AX^H�����,#�r#t�Ȓ�,$k�[
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scripts/static-pie-template-amd64-shorter.rs

@@ -5,4 +5,4 @@
 $$$$solution_src$$$$
 }
 // SOLUTION END
-#[no_link]extern crate std;#[no_mangle]unsafe fn _start(){std::arch::asm!(".quad 56c931459958096ah,6a07b25e016aff31h,0f5841ff6a5a4122h,0e0c11fb097485e05h,74237299232cac0dh,6b242cac92c8fe16h,8e8c1aad0015bc0h,0fdfebf77510c4f6h,0aaf3cfff48ff4fb6h,0e7fff87f2b48d7ebh",in("rsi")r$$$$binary_raw_base91$$$$.as_ptr())}
+#[no_link]extern crate std;#[no_mangle]unsafe fn _start(){std::arch::asm!(".quad 56c931459958096ah,6a07b25e016aff31h,0f5841ff6a5a4122h,0e0c11fb097485e05h,74237299232cac0dh,6b242cac92c8fe16h,8e8c1aad0015bc0h,0fdfebf77510c4f6h,0aaf3cfff48ff4fb6h,0d7fff87f2b48d7ebh",in("rsi")r$$$$binary_raw_base91$$$$.as_ptr())}

src/bin/codegen.rs

@@ -83,17 +83,15 @@ unsafe extern "win64" fn _start() -> ! {
     );
     // For "short", we always assume we are running with loader on Linux,
     // since "short" is only meaningful when submitting to online judges (not local test runs).
+    // Note that the stub will ensure that stack is aligned before caling _start.
+    // Also, for "short" on x86_64 Linux, we don't need PLATFORM_DATA, so we don't fabricate it.
     #[cfg(feature = "short")]
     asm!(
         "clc",                              // Not needed but packager wants it
-        "sub    rsp, 80",                   // 16 + 80 = 96 = 16*6 -> stack alignment preserved
-        "push   1",                         // env_flags = 1 (ENV_FLAGS_LINUX_STYLE_CHKSTK)
-        "push   2",                         // env_id = 2 (ENV_ID_LINUX)
+        "push   rax",                       // Align stack
         "lea    rdi, [rip + __ehdr_start]",
         "lea    rsi, [rip + _DYNAMIC]",
         "call   {0}",
-        "push   rsp",
-        "pop    rcx",
         "call   {1}",                       // This won't return since on Linux we invoke SYS_exitgroup in binary
         sym loader::amd64_elf::relocate,
         sym _start_rust,