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Fri, 24 Jul 2026 04:04:41 -0700 (PDT) Message-ID: <12ca9e3c-3cd7-4081-a906-593dcceca85b@suse.com> Date: Fri, 24 Jul 2026 14:04:39 +0300 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 2/5] x86/boot/64: Clear BSS as early as possible To: Brian Gerst , linux-kernel@vger.kernel.org, x86@kernel.org Cc: Thomas Gleixner , Borislav Petkov , Ard Biesheuvel , Juergen Gross , Tom Lendacky References: <20260724030256.232690-1-brgerst@gmail.com> <20260724030256.232690-3-brgerst@gmail.com> From: Nikolay Borisov Content-Language: en-US Autocrypt: addr=nik.borisov@suse.com; keydata= xsFNBGcrpvIBEAD5cAR5+qu30GnmPrK9veWX5RVzzbgtkk9C/EESHy9Yz0+HWgCVRoNyRQsZ 7DW7vE1KhioDLXjDmeu8/0A8u5nFMqv6d1Gt1lb7XzSAYw7uSWXLPEjFBtz9+fBJJLgbYU7G OpTKy6gRr6GaItZze+r04PGWjeyVUuHZuncTO7B2huxcwIk9tFtRX21gVSOOC96HcxSVVA7X N/LLM2EOL7kg4/yDWEhAdLQDChswhmdpHkp5g6ytj9TM8bNlq9I41hl/3cBEeAkxtb/eS5YR 88LBb/2FkcGnhxkGJPNB+4Siku7K8Mk2Y6elnkOctJcDvk29DajYbQnnW4nhfelZuLNupb1O M0912EvzOVI0dIVgR+xtosp66bYTOpX4Xb0fylED9kYGiuEAeoQZaDQ2eICDcHPiaLzh+6cc pkVTB0sXkWHUsPamtPum6/PgWLE9vGI5s+FaqBaqBYDKyvtJfLK4BdZng0Uc3ijycPs3bpbQ bOnK9LD8TYmYaeTenoNILQ7Ut54CCEXkP446skUMKrEo/HabvkykyWqWiIE/UlAYAx9+Ckho TT1d2QsmsAiYYWwjU8igXBecIbC0uRtF/cTfelNGrQwbICUT6kJjcOTpQDaVyIgRSlUMrlNZ XPVEQ6Zq3/aENA8ObhFxE5PLJPizJH6SC89BMKF3zg6SKx0qzQARAQABzSZOaWtvbGF5IEJv cmlzb3YgPG5pay5ib3Jpc292QHN1c2UuY29tPsLBkQQTAQoAOxYhBDuWB8EJLBUZCPjT3SRn XZEnyhfsBQJnK6byAhsDBQsJCAcCAiICBhUKCQgLAgQWAgMBAh4HAheAAAoJECRnXZEnyhfs XbIQAJxuUnelGdXbSbtovBNm+HF3LtT0XnZ0+DoR0DemUGuA1bZAlaOXGr5mvVbTgaoGUQIJ 3Ejx3UBEG7ZSJcfJobB34w1qHEDO0pN9orGIFT9Bic3lqhawD2r85QMcWwjsZH5FhyRx7P2o DTuUClLMO95GuHYQngBF2rHHl8QMJPVKsR18w4IWAhALpEApxa3luyV7pAAqKllfCNt7tmed uKmclf/Sz6qoP75CvEtRbfAOqYgG1Uk9A62C51iAPe35neMre3WGLsdgyMj4/15jPYi+tOUX Tc7AAWgc95LXyPJo8069MOU73htZmgH4OYy+S7f+ArXD7h8lTLT1niff2bCPi6eiAQq6b5CJ Ka4/27IiZo8tm1XjLYmoBmaCovqx5y5Xt2koibIWG3ZGD2I+qRwZ0UohKRH6kKVHGcrmCv0J YO8yIprxgoYmA7gq21BpTqw3D4+8xujn/6LgndLKmGESM1FuY3ymXgj5983eqaxicKpT9iq8 /a1j31tms4azR7+6Dt8H4SagfN6VbJ0luPzobrrNFxUgpjR4ZyQQ++G7oSRdwjfIh1wuCF6/ mDUNcb6/kA0JS9otiC3omfht47yQnvod+MxFk1lTNUu3hePJUwg1vT1te3vO5oln8lkUo9BU knlYpQ7QA2rDEKs+YWqUstr4pDtHzwQ6mo0rqP+zzsFNBGcrpvIBEADGYTFkNVttZkt6e7yA LNkv3Q39zQCt8qe7qkPdlj3CqygVXfw+h7GlcT9fuc4kd7YxFys4/Wd9icj9ZatGMwffONmi LnUotIq2N7+xvc4Xu76wv+QJpiuGEfCDB+VdZOmOzUPlmMkcJc/EDSH4qGogIYRu72uweKEq VfBI43PZIGpGJ7TjS3THX5WVI2YNSmuwqxnQF/iVqDtD2N72ObkBwIf9GnrOgxEyJ/SQq2R0 g7hd6IYk7SOKt1a8ZGCN6hXXKzmM6gHRC8fyWeTqJcK4BKSdX8PzEuYmAJjSfx4w6DoxdK5/ 9sVrNzaVgDHS0ThH/5kNkZ65KNR7K2nk45LT5Crjbg7w5/kKDY6/XiXDx7v/BOR/a+Ryo+lM MffN3XSnAex8cmIhNINl5Z8CAvDLUtItLcbDOv7hdXt6DSyb65CdyY8JwOt6CWno1tdjyDEG 5ANwVPYY878IFkOJLRTJuUd5ltybaSWjKIwjYJfIXuoyzE7OL63856MC/Os8PcLfY7vYY2LB cvKH1qOcs+an86DWX17+dkcKD/YLrpzwvRMur5+kTgVfXcC0TAl39N4YtaCKM/3ugAaVS1Mw MrbyGnGqVMqlCpjnpYREzapSk8XxbO2kYRsZQd8J9ei98OSqgPf8xM7NCULd/xaZLJUydql1 JdSREId2C15jut21aQARAQABwsF2BBgBCgAgFiEEO5YHwQksFRkI+NPdJGddkSfKF+wFAmcr pvICGwwACgkQJGddkSfKF+xuuxAA4F9iQc61wvAOAidktv4Rztn4QKy8TAyGN3M8zYf/A5Zx VcGgX4J4MhRUoPQNrzmVlrrtE2KILHxQZx5eQyPgixPXri42oG5ePEXZoLU5GFRYSPjjTYmP ypyTPN7uoWLfw4TxJqWCGRLsjnkwvyN3R4161Dty4Uhzqp1IkNhl3ifTDYEvbnmHaNvlvvna 7+9jjEBDEFYDMuO/CA8UtoVQXjy5gtOhZZkEsptfwQYc+E9U99yxGofDul7xH41VdXGpIhUj 4wjd3IbgaCiHxxj/M9eM99ybu5asvHyMo3EFPkyWxZsBlUN/riFXGspG4sT0cwOUhG2ZnExv XXhOGKs/y3VGhjZeCDWZ+0ZQHPCL3HUebLxW49wwLxvXU6sLNfYnTJxdqn58Aq4sBXW5Un0Q vfbd9VFV/bKFfvUscYk2UKPi9vgn1hY38IfmsnoS8b0uwDq75IBvup9pYFyNyPf5SutxhFfP JDjakbdjBoYDWVoaPbp5KAQ2VQRiR54lir/inyqGX+dwzPX/F4OHfB5RTiAFLJliCxniKFsM d8eHe88jWjm6/ilx4IlLl9/MdVUGjLpBi18X7ejLz3U2quYD8DBAGzCjy49wJ4Di4qQjblb2 pTXoEyM2L6E604NbDu0VDvHg7EXh1WwmijEu28c/hEB6DwtzslLpBSsJV0s1/jE= In-Reply-To: <20260724030256.232690-3-brgerst@gmail.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 7/24/26 06:02, Brian Gerst wrote: > Currently, the BSS section is not cleared until x86_64_start_kernel(). > Using unitialized BSS data before that point leads to difficult to debug > problems. Fix this by moving clear_bss() to as early as possible. > > Signed-off-by: Brian Gerst > diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S > index 7ed5520dd52e..15ef5ea52b9a 100644 > --- a/arch/x86/kernel/head_64.S > +++ b/arch/x86/kernel/head_64.S > @@ -37,6 +37,8 @@ > .code64 > SYM_CODE_START_NOALIGN(startup_64) > UNWIND_HINT_END_OF_STACK > + cld > + > /* > * At this point the CPU runs in 64bit mode CS.L = 1 CS.D = 0, > * and someone has loaded an identity mapped page table > @@ -61,6 +63,8 @@ SYM_CODE_START_NOALIGN(startup_64) > /* Set up the stack for verify_cpu() */ > leaq __top_init_kernel_stack(%rip), %rsp > > + call clear_bss > + > /* > * Set up GSBASE. > * Note that on SMP the boot CPU uses the init data section until > @@ -140,6 +144,22 @@ SYM_CODE_START_NOALIGN(startup_64) > jmp *.Lcommon_startup_64(%rip) > SYM_CODE_END(startup_64) > > +SYM_FUNC_START(clear_bss) > + xorl %eax, %eax > + > + leaq __bss_start(%rip), %rdi > + leaq __bss_stop(%rip), %rcx > + subq %rdi, %rcx > + rep stosb Why not stosq (with appropriate adjustment to xor %rax, %rax as well)? The length is guaranteed to be multiple of page_size ergo 8 byte as well ? > + > + leaq __brk_base(%rip), %rdi > + leaq __brk_limit(%rip), %rcx > + subq %rdi, %rcx > + rep stosb > + > + RET > +SYM_FUNC_END(clear_bss) > + > __INITRODATA > SYM_DATA_LOCAL(.Lcommon_startup_64, .quad common_startup_64) > > diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c > index 2c64b388f616..6aa13d19c0a4 100644 > --- a/arch/x86/xen/enlighten_pv.c > +++ b/arch/x86/xen/enlighten_pv.c > @@ -1334,8 +1334,6 @@ asmlinkage __visible void __init xen_start_kernel(struct start_info *si) > if (!si) > return; > > - clear_bss(); > - > xen_start_info = si; > > __text_gen_insn(&early_xen_iret_patch, > diff --git a/arch/x86/xen/xen-head.S b/arch/x86/xen/xen-head.S > index 5dad6c51cdc3..9b56659246fe 100644 > --- a/arch/x86/xen/xen-head.S > +++ b/arch/x86/xen/xen-head.S > @@ -31,6 +31,9 @@ SYM_CODE_START(startup_xen) > > leaq __top_init_kernel_stack(%rip), %rsp > > + movq %rsi, %r15 Why is this change necessary, rsi is ont used in clear_bss ? > + call clear_bss > + > /* > * Set up GSBASE. > * Note that, on SMP, the boot cpu uses init data section until > @@ -41,7 +44,7 @@ SYM_CODE_START(startup_xen) > xorl %edx, %edx > wrmsr > > - mov %rsi, %rdi > + mov %r15, %rdi > call xen_start_kernel > SYM_CODE_END(startup_xen) > __FINIT