aarch64: simplify calls to __libc_arm_za_disable in assembly

There is no functional change in this patch.

We remove stores and loads to stack, return address signing, and redundant
CFI directives before and after call to __libc_arm_za_disable().

The __libc_arm_za_disable implementation follows special calling convention
that allows to avoid most of the operations that would be necessary for a
call to a normal function (see [1] for details).

First, we rely on __libc_arm_za_disable() not clobbering certain registers,
and we put return address into one of these registers. Now we don't need
to store it on stack, so we don't need to sign return address using PAC.

Second, as a result of the above, we don't need to update the CFI offset.

This patch provides small optimisation avoiding unnecessary store and load
on stack also simplifies assembly code and CFI directives.

[1]: https://github.com/ARM-software/abi-aa/blob/main/aapcs64/aapcs64.rst

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

diff --git a/sysdeps/aarch64/__longjmp.S b/sysdeps/aarch64/__longjmp.S
index ed5b6b1fbaf36a642ce815605bbefc1e5fd5d06f..70ac02c44b203179720f786b90de891e0a47028d 100644 (file)
--- a/sysdeps/aarch64/__longjmp.S
+++ b/sysdeps/aarch64/__longjmp.S
@@ -26,21 +26,16 @@
 ENTRY (__longjmp)
 
 #if IS_IN(libc)
-       /* Disable ZA state of SME in libc.a and libc.so, but not in ld.so.  */
-       paciasp
-       cfi_negate_ra_state
-       stp     x29, x30, [sp, -16]!
-       cfi_adjust_cfa_offset (16)
-       cfi_rel_offset (x29, 0)
-       cfi_rel_offset (x30, 8)
-       mov     x29, sp
+       /* Disable ZA state of SME in libc.a and libc.so, but not in ld.so.
+          The calling convention of __libc_arm_za_disable allows to do
+          this thus allowing to avoid saving to and reading from stack.
+          As a result we also don't need to sign the return address and
+          check it after returning because it is not stored to stack.  */
+       mov     x13, x30
+       cfi_register (x30, x13)
        bl      __libc_arm_za_disable
-       ldp     x29, x30, [sp], 16
-       cfi_adjust_cfa_offset (-16)
-       cfi_restore (x29)
-       cfi_restore (x30)
-       autiasp
-       cfi_negate_ra_state
+       mov     x30, x13
+       cfi_register (x13, x30)
 #endif
 
        cfi_def_cfa (x0, 0)

diff --git a/sysdeps/aarch64/setjmp.S b/sysdeps/aarch64/setjmp.S
index c8a2994f4686817aeb8511c93d3a6c4e2fdc1490..53c5e7d8ccb1ed6920e46fd52c8266d91936f386 100644 (file)
--- a/sysdeps/aarch64/setjmp.S
+++ b/sysdeps/aarch64/setjmp.S
@@ -37,21 +37,16 @@ ENTRY_ALIGN (__sigsetjmp, 2)
 1:
 
 #if IS_IN(libc)
-       /* Disable ZA state of SME in libc.a and libc.so, but not in ld.so.  */
-       paciasp
-       cfi_negate_ra_state
-       stp     x29, x30, [sp, -16]!
-       cfi_adjust_cfa_offset (16)
-       cfi_rel_offset (x29, 0)
-       cfi_rel_offset (x30, 8)
-       mov     x29, sp
+       /* Disable ZA state of SME in libc.a and libc.so, but not in ld.so.
+          The calling convention of __libc_arm_za_disable allows to do
+          this thus allowing to avoid saving to and reading from stack.
+          As a result we also don't need to sign the return address and
+          check it after returning because it is not stored to stack.  */
+       mov     x13, x30
+       cfi_register (x30, x13)
        bl      __libc_arm_za_disable
-       ldp     x29, x30, [sp], 16
-       cfi_adjust_cfa_offset (-16)
-       cfi_restore (x29)
-       cfi_restore (x30)
-       autiasp
-       cfi_negate_ra_state
+       mov     x30, x13
+       cfi_register (x13, x30)
 #endif
 
        stp     x19, x20, [x0, #JB_X19<<3]

diff --git a/sysdeps/unix/sysv/linux/aarch64/setcontext.S b/sysdeps/unix/sysv/linux/aarch64/setcontext.S
index 18e4fb7f2103c838aacc6d01f98a272bacbd5381..d9716f012ead35fc474c85424b0cc6a059a8b165 100644 (file)
--- a/sysdeps/unix/sysv/linux/aarch64/setcontext.S
+++ b/sysdeps/unix/sysv/linux/aarch64/setcontext.S
@@ -48,21 +48,16 @@ ENTRY (__setcontext)
        cbz     x0, 1f
        b       C_SYMBOL_NAME (__syscall_error)
 1:
-       /* Disable ZA of SME.  */
-       paciasp
-       cfi_negate_ra_state
-       stp     x29, x30, [sp, -16]!
-       cfi_adjust_cfa_offset (16)
-       cfi_rel_offset (x29, 0)
-       cfi_rel_offset (x30, 8)
-       mov     x29, sp
+       /* Clear ZA state of SME.  */
+       /* The calling convention of __libc_arm_za_disable allows to do
+          this thus allowing to avoid saving to and reading from stack.
+          As a result we also don't need to sign the return address and
+          check it after returning because it is not stored to stack.  */
+       mov     x13, x30
+       cfi_register (x30, x13)
        bl      __libc_arm_za_disable
-       ldp     x29, x30, [sp], 16
-       cfi_adjust_cfa_offset (-16)
-       cfi_restore (x29)
-       cfi_restore (x30)
-       autiasp
-       cfi_negate_ra_state
+       mov     x30, x13
+       cfi_register (x13, x30)
        /* Restore the general purpose registers.  */
        mov     x0, x9
        cfi_def_cfa (x0, 0)