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b=HgT+B5F479qV/5nLGnt80/yCVTIktRnkC03yGxvCIrbZHlezYAQBn++/pM/mbS6nE0qN1Ufnno8XjWzLhLJbpCWtndXDgry+NSciaqhE/S8yXNfYAz55wlEUuLeA9WaCyQw9Dz6MJoNduM2AYqNeOdLLDzRb8bR2lpww9nW3oMI= Authentication-Results: dkim=none (message not signed) header.d=none;dmarc=none action=none header.from=citrix.com; Message-ID: <69ae3671-aa66-4717-91e5-3b989e99c8d3@citrix.com> Date: Mon, 29 Jun 2026 17:33:40 +0100 User-Agent: Mozilla Thunderbird Cc: Andrew Cooper , roger.pau@citrix.com, ross.lagerwall@citrix.com, sstabellini@kernel.org, julien@xen.org, bertrand.marquis@arm.com, michal.orzel@amd.com, Volodymyr_Babchuk@epam.com, anthony.perard@vates.tech, jbeulich@suse.com, taka@valinux.co.jp, den@valinux.co.jp Subject: Re: [RFC] xen/arm64: livepatch: enable attaching callbacks To: Ryo Takakura , xen-devel@lists.xenproject.org References: <20260629020128.30561-1-takakura@valinux.co.jp> Content-Language: en-GB From: Andrew Cooper Autocrypt: addr=andrew.cooper3@citrix.com; keydata= xsFNBFLhNn8BEADVhE+Hb8i0GV6mihnnr/uiQQdPF8kUoFzCOPXkf7jQ5sLYeJa0cQi6Penp VtiFYznTairnVsN5J+ujSTIb+OlMSJUWV4opS7WVNnxHbFTPYZVQ3erv7NKc2iVizCRZ2Kxn srM1oPXWRic8BIAdYOKOloF2300SL/bIpeD+x7h3w9B/qez7nOin5NzkxgFoaUeIal12pXSR Q354FKFoy6Vh96gc4VRqte3jw8mPuJQpfws+Pb+swvSf/i1q1+1I4jsRQQh2m6OTADHIqg2E ofTYAEh7R5HfPx0EXoEDMdRjOeKn8+vvkAwhviWXTHlG3R1QkbE5M/oywnZ83udJmi+lxjJ5 YhQ5IzomvJ16H0Bq+TLyVLO/VRksp1VR9HxCzItLNCS8PdpYYz5TC204ViycobYU65WMpzWe LFAGn8jSS25XIpqv0Y9k87dLbctKKA14Ifw2kq5OIVu2FuX+3i446JOa2vpCI9GcjCzi3oHV e00bzYiHMIl0FICrNJU0Kjho8pdo0m2uxkn6SYEpogAy9pnatUlO+erL4LqFUO7GXSdBRbw5 gNt25XTLdSFuZtMxkY3tq8MFss5QnjhehCVPEpE6y9ZjI4XB8ad1G4oBHVGK5LMsvg22PfMJ ISWFSHoF/B5+lHkCKWkFxZ0gZn33ju5n6/FOdEx4B8cMJt+cWwARAQABzSlBbmRyZXcgQ29v cGVyIDxhbmRyZXcuY29vcGVyM0BjaXRyaXguY29tPsLBegQTAQgAJAIbAwULCQgHAwUVCgkI CwUWAgMBAAIeAQIXgAUCWKD95wIZAQAKCRBlw/kGpdefoHbdD/9AIoR3k6fKl+RFiFpyAhvO 59ttDFI7nIAnlYngev2XUR3acFElJATHSDO0ju+hqWqAb8kVijXLops0gOfqt3VPZq9cuHlh IMDquatGLzAadfFx2eQYIYT+FYuMoPZy/aTUazmJIDVxP7L383grjIkn+7tAv+qeDfE+txL4 SAm1UHNvmdfgL2/lcmL3xRh7sub3nJilM93RWX1Pe5LBSDXO45uzCGEdst6uSlzYR/MEr+5Z JQQ32JV64zwvf/aKaagSQSQMYNX9JFgfZ3TKWC1KJQbX5ssoX/5hNLqxMcZV3TN7kU8I3kjK mPec9+1nECOjjJSO/h4P0sBZyIUGfguwzhEeGf4sMCuSEM4xjCnwiBwftR17sr0spYcOpqET ZGcAmyYcNjy6CYadNCnfR40vhhWuCfNCBzWnUW0lFoo12wb0YnzoOLjvfD6OL3JjIUJNOmJy RCsJ5IA/Iz33RhSVRmROu+TztwuThClw63g7+hoyewv7BemKyuU6FTVhjjW+XUWmS/FzknSi dAG+insr0746cTPpSkGl3KAXeWDGJzve7/SBBfyznWCMGaf8E2P1oOdIZRxHgWj0zNr1+ooF /PzgLPiCI4OMUttTlEKChgbUTQ+5o0P080JojqfXwbPAyumbaYcQNiH1/xYbJdOFSiBv9rpt TQTBLzDKXok86M7BTQRS4TZ/ARAAkgqudHsp+hd82UVkvgnlqZjzz2vyrYfz7bkPtXaGb9H4 Rfo7mQsEQavEBdWWjbga6eMnDqtu+FC+qeTGYebToxEyp2lKDSoAsvt8w82tIlP/EbmRbDVn 7bhjBlfRcFjVYw8uVDPptT0TV47vpoCVkTwcyb6OltJrvg/QzV9f07DJswuda1JH3/qvYu0p vjPnYvCq4NsqY2XSdAJ02HrdYPFtNyPEntu1n1KK+gJrstjtw7KsZ4ygXYrsm/oCBiVW/OgU g/XIlGErkrxe4vQvJyVwg6YH653YTX5hLLUEL1NS4TCo47RP+wi6y+TnuAL36UtK/uFyEuPy wwrDVcC4cIFhYSfsO0BumEI65yu7a8aHbGfq2lW251UcoU48Z27ZUUZd2Dr6O/n8poQHbaTd 6bJJSjzGGHZVbRP9UQ3lkmkmc0+XCHmj5WhwNNYjgbbmML7y0fsJT5RgvefAIFfHBg7fTY/i kBEimoUsTEQz+N4hbKwo1hULfVxDJStE4sbPhjbsPCrlXf6W9CxSyQ0qmZ2bXsLQYRj2xqd1 bpA+1o1j2N4/au1R/uSiUFjewJdT/LX1EklKDcQwpk06Af/N7VZtSfEJeRV04unbsKVXWZAk uAJyDDKN99ziC0Wz5kcPyVD1HNf8bgaqGDzrv3TfYjwqayRFcMf7xJaL9xXedMcAEQEAAcLB XwQYAQgACQUCUuE2fwIbDAAKCRBlw/kGpdefoG4XEACD1Qf/er8EA7g23HMxYWd3FXHThrVQ HgiGdk5Yh632vjOm9L4sd/GCEACVQKjsu98e8o3ysitFlznEns5EAAXEbITrgKWXDDUWGYxd pnjj2u+GkVdsOAGk0kxczX6s+VRBhpbBI2PWnOsRJgU2n10PZ3mZD4Xu9kU2IXYmuW+e5KCA vTArRUdCrAtIa1k01sPipPPw6dfxx2e5asy21YOytzxuWFfJTGnVxZZSCyLUO83sh6OZhJkk b9rxL9wPmpN/t2IPaEKoAc0FTQZS36wAMOXkBh24PQ9gaLJvfPKpNzGD8XWR5HHF0NLIJhgg 4ZlEXQ2fVp3XrtocHqhu4UZR4koCijgB8sB7Tb0GCpwK+C4UePdFLfhKyRdSXuvY3AHJd4CP 4JzW0Bzq/WXY3XMOzUTYApGQpnUpdOmuQSfpV9MQO+/jo7r6yPbxT7CwRS5dcQPzUiuHLK9i nvjREdh84qycnx0/6dDroYhp0DFv4udxuAvt1h4wGwTPRQZerSm4xaYegEFusyhbZrI0U9tJ B8WrhBLXDiYlyJT6zOV2yZFuW47VrLsjYnHwn27hmxTC/7tvG3euCklmkn9Sl9IAKFu29RSo d5bD8kMSCYsTqtTfT6W4A3qHGvIDta3ptLYpIAOD2sY3GYq2nf3Bbzx81wZK14JdDDHUX2Rs 6+ahAA== In-Reply-To: <20260629020128.30561-1-takakura@valinux.co.jp> Content-Type: text/plain; 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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 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