setcontext returns to the specified context via an indirect jump,
so there should be a BTI j.
In case of getcontext (and all other returns_twice functions) the
compiler adds BTI j at the call site, but swapcontext is a normal
c call that is currently not handled specially by the compiler.
So we change swapcontext such that the saved context returns to a
local address that has BTI j and then swapcontext returns to the
caller via a normal RET. For this we save the original return
address in the slot for x1 of the context because x1 need not be
preserved by swapcontext but it is restored when the context saved
by swapcontext is resumed.
The alternative fix (which is done on x86) would make swapcontext
special in the compiler so BTI j is emitted at call sites, on
x86 there is an indirect_return attribute for this, on AArch64
we would have to use returns_twice. It was decided against because
such fix may need user code updates: the attribute has to be added
when swapcontext is called via a function pointer and it breaks
always_inline functions with swapcontext.
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
since
https://sourceware.org/ml/libc-alpha/2014-04/msg00006.html
setcontext etc is no longer tied to the kernel use of ucontext.
in that patch the ucontext reserved space is not used consistently
with the kernel abi: the d8,d9 pair is saved in the slot of q8.
this is ok (*context functions work together), but probably not
desirable (ucontexts created by the kernel and getcontext are
subtly different).
the fix just replaces dN with qN in the save/restore code, which
does a bit more than needed (saves/restores the top half of qN that
is not callee saved), but this should not be an issue (and avoids
having to deal with endianness).
(kernel fpsimd context layout: the first 64bit contains 0x210 the fpsimd
context size and 0x46508001 the FPSIMD_MAGIC, the second 64bit is for
fpsr and fpcr, and the rest is the 128bit q0..q31 registers).
given d8=8.1, d9=9.1,... d15=15.1, the context created by getcontext is
current:
(gdb) x/40xg ctx.uc_mcontext.__reserved
0x410df0 <ctx+464>: 0x0000021046508001 0x0000000000000000
0x410e00 <ctx+480>: 0x0000000000000000 0x0000000000000000
0x410e10 <ctx+496>: 0x0000000000000000 0x0000000000000000
0x410e20 <ctx+512>: 0x0000000000000000 0x0000000000000000
0x410e30 <ctx+528>: 0x0000000000000000 0x0000000000000000
0x410e40 <ctx+544>: 0x0000000000000000 0x0000000000000000
0x410e50 <ctx+560>: 0x0000000000000000 0x0000000000000000
0x410e60 <ctx+576>: 0x0000000000000000 0x0000000000000000
0x410e70 <ctx+592>: 0x0000000000000000 0x0000000000000000
0x410e80 <ctx+608>: 0x4020333333333333 0x4022333333333333
0x410e90 <ctx+624>: 0x0000000000000000 0x0000000000000000
0x410ea0 <ctx+640>: 0x4024333333333333 0x4026333333333333
0x410eb0 <ctx+656>: 0x0000000000000000 0x0000000000000000
0x410ec0 <ctx+672>: 0x4028333333333333 0x402a333333333333
0x410ed0 <ctx+688>: 0x0000000000000000 0x0000000000000000
0x410ee0 <ctx+704>: 0x402c333333333333 0x402e333333333333
0x410ef0 <ctx+720>: 0x0000000000000000 0x0000000000000000
0x410f00 <ctx+736>: 0x0000000000000000 0x0000000000000000
0x410f10 <ctx+752>: 0x0000000000000000 0x0000000000000000
0x410f20 <ctx+768>: 0x0000000000000000 0x0000000000000000
fixed:
(gdb) x/40xg ctx.uc_mcontext.__reserved
0x410d70 <ctx+464>: 0x0000021046508001 0x0000000000000000
0x410d80 <ctx+480>: 0x0000000000000000 0x0000000000000000
0x410d90 <ctx+496>: 0x0000000000000000 0x0000000000000000
0x410da0 <ctx+512>: 0x0000000000000000 0x0000000000000000
0x410db0 <ctx+528>: 0x0000000000000000 0x0000000000000000
0x410dc0 <ctx+544>: 0x0000000000000000 0x0000000000000000
0x410dd0 <ctx+560>: 0x0000000000000000 0x0000000000000000
0x410de0 <ctx+576>: 0x0000000000000000 0x0000000000000000
0x410df0 <ctx+592>: 0x0000000000000000 0x0000000000000000
0x410e00 <ctx+608>: 0x4020333333333333 0x0000000000000000
0x410e10 <ctx+624>: 0x4022333333333333 0x0000000000000000
0x410e20 <ctx+640>: 0x4024333333333333 0x0000000000000000
0x410e30 <ctx+656>: 0x4026333333333333 0x0000000000000000
0x410e40 <ctx+672>: 0x4028333333333333 0x0000000000000000
0x410e50 <ctx+688>: 0x402a333333333333 0x0000000000000000
0x410e60 <ctx+704>: 0x402c333333333333 0x0000000000000000
0x410e70 <ctx+720>: 0x402e333333333333 0x0000000000000000
0x410e80 <ctx+736>: 0x0000000000000000 0x0000000000000000
0x410e90 <ctx+752>: 0x0000000000000000 0x0000000000000000
0x410ea0 <ctx+768>: 0x0000000000000000 0x0000000000000000
2015-07-06 Szabolcs Nagy <szabolcs.nagy@arm.com>
* sysdeps/unix/sysv/linux/aarch64/getcontext.S (__getcontext): Use q
registers instead of d ones so the layout is kernel abi compatible.
* sysdeps/unix/sysv/linux/aarch64/setcontext.S (__setcontext): Likewise.
* sysdeps/unix/sysv/linux/aarch64/swapcontext.S (__swapcontext):
Likewise.# Please enter the commit message for your changes. Lines starting
This patch moves the AArch64 port to the main sysdeps hierarchy. The
move is essentially:
git mv ports/sysdeps/aarch64 sysdeps/aarch64
git mv ports/sysdeps/unix/sysv/linux/aarch64 sysdeps/unix/sysv/linux/aarch64
The README is updated and I've updated ChangeLog.aarch64 along the
lines of the ARM move. The AArch64 build has been tested to confirm
that there were no changes in objdump -dr output or the shared
objects.
