On 3/18/2020 6:11 PM, Sean Christopherson wrote:
Make __vdso_sgx_enter_enclave() callable from C by preserving %rbx
and taking @leaf in %rcx instead of %rax. Being able to invoke the vDSO
from C reduces the overhead of runtimes that are tightly coupled with
their enclaves, e.g. that can rely on the enclave to save and restore
non-volatile registers, as the runtime doesn't need an assembly wrapper
to preserve non-volatile registers and/or shuffle stack arguments.
Note, both %rcx and %rbx are consumed by EENTER/ERESUME, i.e. consuming
them doesn't violate the primary tenet of __vdso_sgx_enter_enclave()
that "thou shalt not restrict how information is exchanged between an
enclave and its host process".
Suggested-by: Nathaniel McCallum <npmccallum@xxxxxxxxxx>
Cc: Cedric Xing <cedric.xing@xxxxxxxxx>
Cc: Jethro Beekman <jethro@xxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: linux-sgx@xxxxxxxxxxxxxxx
Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
---
Cedric, please go to town on this, you're much better at the userspace
stack and debugging interactions, e.g. I don't think the CFA directives
need updating, but my knowledge of that stuff is skeeetchy.
Note, if the previous patch to make the %rsp restoration relative is
dropped, then %rbx should be restored relative to %rbp instead of %rsp.
arch/x86/entry/vdso/vsgx_enter_enclave.S | 33 ++++++++++++++----------
1 file changed, 19 insertions(+), 14 deletions(-)
diff --git a/arch/x86/entry/vdso/vsgx_enter_enclave.S b/arch/x86/entry/vdso/vsgx_enter_enclave.S
index 14f07d5e47ae..7a0565476a29 100644
--- a/arch/x86/entry/vdso/vsgx_enter_enclave.S
+++ b/arch/x86/entry/vdso/vsgx_enter_enclave.S
@@ -16,25 +16,25 @@
/**
* __vdso_sgx_enter_enclave() - Enter an SGX enclave
+ * @rdi: Pass-through value for RDI
+ * @rsi: Pass-through value for RSI
+ * @rdx: Pass-through value for RDX
* @leaf: ENCLU leaf, must be EENTER or ERESUME
+ * @r8: Pass-through value for R8
+ * @r9: Pass-through value for R9
* @tcs: TCS, must be non-NULL
* @e: Optional struct sgx_enclave_exception instance
* @handler: Optional enclave exit handler
*
- * **Important!** __vdso_sgx_enter_enclave() is **NOT** compliant with the
- * x86-64 ABI, i.e. cannot be called from standard C code.
- *
- * Input ABI:
- * @leaf %eax
- * @tcs 8(%rsp)
- * @e 0x10(%rsp)
- * @handler 0x18(%rsp)
- *
- * Output ABI:
- * @ret %eax
+ * **Important!** __vdso_sgx_enter_enclave() does not ensure full compliance
+ * with the x86-64 ABI, e.g. doesn't explicitly clear EFLAGS.DF after EEXIT.
+ * Except for non-volatile general purpose registers, preserving/setting state
+ * in accordance with the x86-64 ABI is the responsibility of the enclave and
+ * its runtime, i.e. __vdso_sgx_enter_enclave() cannot be called from C code
+ * without careful consideration by both the enclave and its runtime.
*
* All general purpose registers except RAX, RBX and RCX are passed as-is to
- * the enclave. RAX, RBX and RCX are consumed by EENTER and ERESUME and are
+ * the enclave. RAX, RBX and RCX are consumed by EENTER and ERESUME and are
* loaded with @leaf, asynchronous exit pointer, and @tcs respectively.
*
* RBP and the stack are used to anchor __vdso_sgx_enter_enclave() to the
@@ -70,9 +70,11 @@
*/
#ifdef SGX_KERNEL_DOC
/* C-style function prototype to coerce kernel-doc into parsing the comment. */
-int __vdso_sgx_enter_enclave(int leaf, void *tcs,
+int __vdso_sgx_enter_enclave(unsigned long rdi, unsigned long rsi,
+ unsigned long rdx, unsigned int leaf,
+ unsigned long r8, unsigned long r9, void *tcs,
struct sgx_enclave_exception *e,
- sgx_enclave_exit_handler_t handler);
+ sgx_enclave_exit_handler_t handler)
#endif
SYM_FUNC_START(__vdso_sgx_enter_enclave)
/* Prolog */
@@ -82,7 +84,9 @@ SYM_FUNC_START(__vdso_sgx_enter_enclave)
.cfi_rel_offset %rbp, 0
mov %rsp, %rbp
.cfi_def_cfa_register %rbp
+ push %rbx
A CFI directive is needed here:
.cfi_rel_offset %rbx, -8
+ mov %ecx, %eax
.Lenter_enclave:
/* EENTER <= leaf <= ERESUME */
cmp $0x2, %eax
@@ -108,6 +112,7 @@ SYM_FUNC_START(__vdso_sgx_enter_enclave)
jne .Linvoke_userspace_handler
.Lout:
+ pop %rbx
leave
.cfi_def_cfa %rsp, 8
ret
