Re: [RFC] xen/arm64: livepatch: enable attaching callbacks

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 29/06/2026 3:01 am, Ryo Takakura wrote:
> Linux ftrace allows registering callbacks which is useful
> for debugging and tracing events. On Linux, it is done by
> reserving function entry points at compile time which can
> later be patched to branch to a trampoline.
>
> This patch implements similar callback feature, but with
> different approach using existing livepatch infrastructure.
> Instead of reserving function entry points at compile time,
> the traced function will be livepatched so that it branches
> to the trampoline.
>
> The role of the trampoline(illustrated below) is to preserve
> the context while jumping to the tracer function, and return
> back to the traced function with its context restored.
>
> trampoline:
>     Save regs
>     Call tracer function
>     Restore regs
>     old_addr
>     return old_addr + 4
>
> One can request the feature by setting @trampoline_buf to 1
> which will allocate a buffer for trampoline.
>
> Signed-off-by: Ryo Takakura <takakura@xxxxxxxxxxxxx>

Having something a bit more like Linux tracing would be nice.  But, this
is very different to the other livepatching functionality and a few bits
don't match nicely.

First, you write a lot of the trampoline manually.  You can do most of
this in the target function with
__attribute__((no_caller_saved_registers)), avoiding the need to do it
by hand.   This would require a minimum GCC of 7 (where our baseline is
5) but it's acceptable for new features to require a newer compiler.

Secondly, what happens if the instruction at old_addr is an ADRP, or a
branch?  Right now, there's no case where we move an instruction; we
only produce new code, and branch from old to new.

When you're moving the instruction at old_addr, you must compensate for
any IP-relative component.  Also you can in principle have a conditional
branch as the first instruction, which gives you two branches to fix up
at the end of the trampoline, rather than one.

On x86, you've got an additional problem that it's generally more than
one instruction, and rarely an exect number of instructions overwritten
at old_addr.

Some high level comments, leaving aside the details until the above
questions are better understood.

> diff --git a/xen/arch/arm/arm64/livepatch.c b/xen/arch/arm/arm64/livepatch.c
> index e135bd5bf9..b7c9aba94e 100644
> --- a/xen/arch/arm/arm64/livepatch.c
> +++ b/xen/arch/arm/arm64/livepatch.c
> @@ -34,12 +57,87 @@ void arch_livepatch_apply(const struct livepatch_func 
> *func,
>      /* Save old ones. */
>      memcpy(state->insn_buffer, func->old_addr, len);
>  
> -    if ( func->new_addr )
> +    if ( !func->new_addr )
> +    {
> +        insn = aarch64_insn_gen_nop();
> +    }
> +    else if ( func->trampoline_buf )
> +    {
> +        int rc;
> +        uint32_t *trampoline = func->trampoline_buf;
> +        uint32_t *tp = trampoline;
> +        void *orig_cont_addr = (void *)func->old_addr + len;
> +        unsigned int trampoline_code_size = len + 12 * ARCH_PATCH_INSN_SIZE;
> +        unsigned long trampoline_start = (unsigned long)trampoline & 
> PAGE_MASK;
> +        unsigned long trampoline_end =
> +            PAGE_ALIGN((unsigned long)trampoline + trampoline_code_size);
> +
> +        /*
> +         * Make the payload text area writeable while generating
> +         * the trampoline instructions.
> +         */
> +        rc = modify_xen_mappings(trampoline_start, trampoline_end,
> +                                 PAGE_HYPERVISOR);
> +        if ( rc )
> +        {
> +            printk(XENLOG_ERR LIVEPATCH
> +                   "Failed to make trampoline writable: %d\n", rc);
> +            return;
> +        }

This ought not to be necessary.

The trampoline is executable code, so should have space reserved for it
in .text of the livepatch.

Then, you can identify it simply by references in a new section, without
having to have a pointer with a sentinel value (void *)1 in (which MISRA
will have a fit at).

> +
> +        /* Save state before calling the tracer. */
> +        *tp++ = aarch64_insn_gen_stp_pre(0, 1);
> +        *tp++ = aarch64_insn_gen_stp_pre(2, 3);
> +        *tp++ = aarch64_insn_gen_stp_pre(4, 5);
> +        *tp++ = aarch64_insn_gen_stp_pre(6, 7);
> +        *tp++ = aarch64_insn_gen_stp_pre(29, 30);
> +
> +        /* Call user's tracing function. */
> +        insn = aarch64_insn_gen_branch_imm(
> +            (unsigned long)tp,
> +            (unsigned long)func->new_addr,
> +            AARCH64_INSN_BRANCH_LINK);
> +        *tp++ = insn;
> +
> +        /* Restore state before continuing original function. */
> +        *tp++ = aarch64_insn_gen_ldp_post(29, 30);
> +        *tp++ = aarch64_insn_gen_ldp_post(6, 7);
> +        *tp++ = aarch64_insn_gen_ldp_post(4, 5);
> +        *tp++ = aarch64_insn_gen_ldp_post(2, 3);
> +        *tp++ = aarch64_insn_gen_ldp_post(0, 1);
> +
> +        /* Original instruction. */
> +        memcpy(tp, state->insn_buffer, len);
> +        tp += len / ARCH_PATCH_INSN_SIZE;
> +
> +        /* Branch back to original function. */
> +        insn = aarch64_insn_gen_branch_imm(
> +            (unsigned long)tp,
> +            (unsigned long)orig_cont_addr,
> +            AARCH64_INSN_BRANCH_NOLINK);
> +        *tp++ = insn;
> +
> +        clean_and_invalidate_dcache_va_range(trampoline, 
> trampoline_code_size);
> +
> +        rc = modify_xen_mappings(trampoline_start, trampoline_end,
> +                                 PAGE_HYPERVISOR_RX);
> +        if ( rc )
> +        {
> +            printk(XENLOG_ERR LIVEPATCH
> +                   "Failed to restore trampoline RX mapping: %d\n", rc);
> +            return;
> +        }
> +
> +        /* Branch from original function to trampoline. */
> +        insn = aarch64_insn_gen_branch_imm(
> +            (unsigned long)func->old_addr,
> +            (unsigned long)func->trampoline_buf,
> +            AARCH64_INSN_BRANCH_NOLINK);

This entire block wants breaking out into a function for writing the
trampoline.  It does not want to live inline in arch_livepatch_apply().

> diff --git a/xen/include/xen/livepatch.h b/xen/include/xen/livepatch.h
> index 45c8924f34..7a81763cf2 100644
> --- a/xen/include/xen/livepatch.h
> +++ b/xen/include/xen/livepatch.h
> @@ -48,6 +48,8 @@ struct xen_sysctl_livepatch_op;
>  #define ELF_LIVEPATCH_POSTREVERT_HOOK ".livepatch.hooks.postrevert"
>  /* Arbitrary limit for payload size and .bss section size. */
>  #define LIVEPATCH_MAX_SIZE     MB(2)
> +/* Size of a trampoline used for function tracing */
> +#define LIVEPATCH_TRAMPOLINE_SIZE 128

This is a common header.  How have you calculate 128?

At best, it's an Aarch64 specific number, but if you reserve space
properly in .text then it won't even matter, I don't think.

~Andrew