aarch64: fix RTLD_START for BTI

Tailcalls must use x16 or x17 for the indirect branch instruction
to be compatible with code that uses BTI c at function entries.
(Other forms of indirect branches can only land on BTI j.)

Also added a BTI c at the ELF entry point of rtld, this is not
strictly necessary since the kernel does not use indirect branch
to get there, but it seems safest once building glibc itself with
BTI is supported.

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

diff --git a/sysdeps/aarch64/dl-machine.h b/sysdeps/aarch64/dl-machine.h
index db3335e5ad7d406b42d5acbaa5483732501b323c..70b9ed39250a9fd8ab6270940f2f69a383b081a9 100644 (file)
--- a/sysdeps/aarch64/dl-machine.h
+++ b/sysdeps/aarch64/dl-machine.h
@@ -125,6 +125,8 @@ elf_machine_runtime_setup (struct link_map *l, int lazy, int profile)
 .globl _dl_start_user                                                  \n\
 .type _dl_start_user, %function                                                \n\
 _start:                                                                        \n\
+       // bti c                                                        \n\
+       hint    34                                                      \n\
        mov     " PTR "0, " PTR_SP "                                    \n\
        bl      _dl_start                                               \n\
        // returns user entry point in x0                               \n\
@@ -178,7 +180,8 @@ _dl_start_user:                                                             \n\
        adrp    x0, _dl_fini                                            \n\
        add     " PTR "0, " PTR "0, #:lo12:_dl_fini                     \n\
        // jump to the user_s entry point                               \n\
-       br      x21                                                     \n\
+       mov     x16, x21                                                \n\
+       br      x16                                                     \n\
 ");
 
 #define elf_machine_type_class(type)                                   \