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Message-ID: <CAFULd4YA0g0Mx0AKBfM-nK_OuTf0pY_fAnR5CCiJeh0cKPLMnQ@mail.gmail.com>
Date: Thu, 7 Nov 2024 12:28:42 +0100
From: Uros Bizjak <ubizjak@...il.com>
To: Brian Gerst <brgerst@...il.com>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org,
Ingo Molnar <mingo@...nel.org>, "H . Peter Anvin" <hpa@...or.com>, Thomas Gleixner <tglx@...utronix.de>,
Borislav Petkov <bp@...en8.de>, Ard Biesheuvel <ardb@...nel.org>
Subject: Re: [PATCH v5 09/16] x86/percpu/64: Use relative percpu offsets
On Tue, Nov 5, 2024 at 4:58 PM Brian Gerst <brgerst@...il.com> wrote:
>
> The percpu section is currently linked at absolute address 0, because
> older compilers hardcoded the stack protector canary value at a fixed
> offset from the start of the GS segment. Now that the canary is a
> normal percpu variable, the percpu section does not need to be linked
> at a specific address.
>
> x86-64 will now calculate the percpu offsets as the delta between the
> initial percpu address and the dynamically allocated memory, like other
> architectures. Note that GSBASE is limited to the canonical address
> width (48 or 57 bits, sign-extended). As long as the kernel text,
> modules, and the dynamically allocated percpu memmory are all in the
> negative address space, the delta will not overflow this limit.
>
> Signed-off-by: Brian Gerst <brgerst@...il.com>
> ---
> arch/x86/include/asm/processor.h | 6 +++++-
> arch/x86/kernel/head_64.S | 19 +++++++++----------
> arch/x86/kernel/setup_percpu.c | 12 ++----------
> arch/x86/kernel/vmlinux.lds.S | 29 +----------------------------
> arch/x86/platform/pvh/head.S | 5 ++---
> arch/x86/tools/relocs.c | 10 +++-------
> arch/x86/xen/xen-head.S | 9 ++++-----
> init/Kconfig | 2 +-
> 8 files changed, 27 insertions(+), 65 deletions(-)
>
> diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
> index a113c3f4f558..ae50d5d4fa26 100644
> --- a/arch/x86/include/asm/processor.h
> +++ b/arch/x86/include/asm/processor.h
> @@ -428,7 +428,11 @@ DECLARE_INIT_PER_CPU(fixed_percpu_data);
>
> static inline unsigned long cpu_kernelmode_gs_base(int cpu)
> {
> - return (unsigned long)per_cpu(fixed_percpu_data.gs_base, cpu);
> +#ifdef CONFIG_SMP
> + return per_cpu_offset(cpu);
> +#else
> + return 0;
> +#endif
> }
>
> extern asmlinkage void entry_SYSCALL32_ignore(void);
> diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
> index c3028b4df85f..ffbcb0aea450 100644
> --- a/arch/x86/kernel/head_64.S
> +++ b/arch/x86/kernel/head_64.S
> @@ -61,11 +61,14 @@ SYM_CODE_START_NOALIGN(startup_64)
> /* Set up the stack for verify_cpu() */
> leaq __top_init_kernel_stack(%rip), %rsp
>
> - /* Setup GSBASE to allow stack canary access for C code */
> + /*
> + * Set up GSBASE.
> + * Note that, on SMP, the boot cpu uses init data section until
> + * the per cpu areas are set up.
> + */
> movl $MSR_GS_BASE, %ecx
> - leaq INIT_PER_CPU_VAR(fixed_percpu_data)(%rip), %rdx
> - movl %edx, %eax
> - shrq $32, %rdx
> + xorl %eax, %eax
> + xorl %edx, %edx
You can use cltd after "xor %eax, %eax", it is one byte shorter with
the same effect ...
> wrmsr
>
> call startup_64_setup_gdt_idt
> @@ -353,16 +356,12 @@ SYM_INNER_LABEL(common_startup_64, SYM_L_LOCAL)
> movl %eax,%fs
> movl %eax,%gs
>
> - /* Set up %gs.
> - *
> - * The base of %gs always points to fixed_percpu_data.
> + /*
> + * Set up GSBASE.
> * Note that, on SMP, the boot cpu uses init data section until
> * the per cpu areas are set up.
> */
> movl $MSR_GS_BASE,%ecx
> -#ifndef CONFIG_SMP
> - leaq INIT_PER_CPU_VAR(fixed_percpu_data)(%rip), %rdx
> -#endif
> movl %edx, %eax
> shrq $32, %rdx
> wrmsr
> diff --git a/arch/x86/kernel/setup_percpu.c b/arch/x86/kernel/setup_percpu.c
> index b30d6e180df7..1e7be9409aa2 100644
> --- a/arch/x86/kernel/setup_percpu.c
> +++ b/arch/x86/kernel/setup_percpu.c
> @@ -23,18 +23,10 @@
> #include <asm/cpumask.h>
> #include <asm/cpu.h>
>
> -#ifdef CONFIG_X86_64
> -#define BOOT_PERCPU_OFFSET ((unsigned long)__per_cpu_load)
> -#else
> -#define BOOT_PERCPU_OFFSET 0
> -#endif
> -
> -DEFINE_PER_CPU_READ_MOSTLY(unsigned long, this_cpu_off) = BOOT_PERCPU_OFFSET;
> +DEFINE_PER_CPU_READ_MOSTLY(unsigned long, this_cpu_off);
> EXPORT_PER_CPU_SYMBOL(this_cpu_off);
>
> -unsigned long __per_cpu_offset[NR_CPUS] __ro_after_init = {
> - [0 ... NR_CPUS-1] = BOOT_PERCPU_OFFSET,
> -};
> +unsigned long __per_cpu_offset[NR_CPUS] __ro_after_init;
> EXPORT_SYMBOL(__per_cpu_offset);
>
> /*
> diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
> index d61c3584f3e6..42d1c05b0207 100644
> --- a/arch/x86/kernel/vmlinux.lds.S
> +++ b/arch/x86/kernel/vmlinux.lds.S
> @@ -99,12 +99,6 @@ const_pcpu_hot = pcpu_hot;
> PHDRS {
> text PT_LOAD FLAGS(5); /* R_E */
> data PT_LOAD FLAGS(6); /* RW_ */
> -#ifdef CONFIG_X86_64
> -#ifdef CONFIG_SMP
> - percpu PT_LOAD FLAGS(6); /* RW_ */
> -#endif
> - init PT_LOAD FLAGS(7); /* RWE */
> -#endif
> note PT_NOTE FLAGS(0); /* ___ */
> }
>
> @@ -199,21 +193,7 @@ SECTIONS
> __init_begin = .; /* paired with __init_end */
> }
>
> -#if defined(CONFIG_X86_64) && defined(CONFIG_SMP)
> - /*
> - * percpu offsets are zero-based on SMP. PERCPU_VADDR() changes the
> - * output PHDR, so the next output section - .init.text - should
> - * start another segment - init.
> - */
> - PERCPU_VADDR(INTERNODE_CACHE_BYTES, 0, :percpu)
> - ASSERT(SIZEOF(.data..percpu) < CONFIG_PHYSICAL_START,
> - "per-CPU data too large - increase CONFIG_PHYSICAL_START")
> -#endif
> -
> INIT_TEXT_SECTION(PAGE_SIZE)
> -#ifdef CONFIG_X86_64
> - :init
> -#endif
>
> /*
> * Section for code used exclusively before alternatives are run. All
> @@ -330,9 +310,7 @@ SECTIONS
> EXIT_DATA
> }
>
> -#if !defined(CONFIG_X86_64) || !defined(CONFIG_SMP)
> PERCPU_SECTION(INTERNODE_CACHE_BYTES)
> -#endif
>
> RUNTIME_CONST_VARIABLES
> RUNTIME_CONST(ptr, USER_PTR_MAX)
> @@ -476,16 +454,11 @@ PROVIDE(__ref_stack_chk_guard = __stack_chk_guard);
> * Per-cpu symbols which need to be offset from __per_cpu_load
> * for the boot processor.
> */
> -#define INIT_PER_CPU(x) init_per_cpu__##x = ABSOLUTE(x) + __per_cpu_load
> +#define INIT_PER_CPU(x) init_per_cpu__##x = ABSOLUTE(x)
> INIT_PER_CPU(gdt_page);
> INIT_PER_CPU(fixed_percpu_data);
> INIT_PER_CPU(irq_stack_backing_store);
>
> -#ifdef CONFIG_SMP
> -. = ASSERT((fixed_percpu_data == 0),
> - "fixed_percpu_data is not at start of per-cpu area");
> -#endif
> -
> #ifdef CONFIG_MITIGATION_UNRET_ENTRY
> . = ASSERT((retbleed_return_thunk & 0x3f) == 0, "retbleed_return_thunk not cacheline-aligned");
> #endif
> diff --git a/arch/x86/platform/pvh/head.S b/arch/x86/platform/pvh/head.S
> index b0a9a58952aa..c931e680ef15 100644
> --- a/arch/x86/platform/pvh/head.S
> +++ b/arch/x86/platform/pvh/head.S
> @@ -165,9 +165,8 @@ SYM_CODE_START_LOCAL(pvh_start_xen)
> * the per cpu areas are set up.
> */
> movl $MSR_GS_BASE,%ecx
> - leaq INIT_PER_CPU_VAR(fixed_percpu_data)(%rip), %rdx
> - movq %edx, %eax
> - shrq $32, %rdx
> + xorl %eax, %eax
> + xorl %edx, %edx
... also here ...
> wrmsr
>
> /*
> diff --git a/arch/x86/tools/relocs.c b/arch/x86/tools/relocs.c
> index 7d7fc7f0a250..8b5e2bc3d241 100644
> --- a/arch/x86/tools/relocs.c
> +++ b/arch/x86/tools/relocs.c
> @@ -834,12 +834,7 @@ static void percpu_init(void)
> */
> static int is_percpu_sym(ElfW(Sym) *sym, const char *symname)
> {
> - int shndx = sym_index(sym);
> -
> - return (shndx == per_cpu_shndx) &&
> - strcmp(symname, "__init_begin") &&
> - strcmp(symname, "__per_cpu_load") &&
> - strncmp(symname, "init_per_cpu_", 13);
> + return 0;
> }
>
>
> @@ -1055,7 +1050,8 @@ static int cmp_relocs(const void *va, const void *vb)
>
> static void sort_relocs(struct relocs *r)
> {
> - qsort(r->offset, r->count, sizeof(r->offset[0]), cmp_relocs);
> + if (r->count)
> + qsort(r->offset, r->count, sizeof(r->offset[0]), cmp_relocs);
> }
>
> static int write32(uint32_t v, FILE *f)
> diff --git a/arch/x86/xen/xen-head.S b/arch/x86/xen/xen-head.S
> index ae4672ea00bb..1796884b727d 100644
> --- a/arch/x86/xen/xen-head.S
> +++ b/arch/x86/xen/xen-head.S
> @@ -51,15 +51,14 @@ SYM_CODE_START(startup_xen)
>
> leaq __top_init_kernel_stack(%rip), %rsp
>
> - /* Set up %gs.
> - *
> - * The base of %gs always points to fixed_percpu_data.
> + /*
> + * Set up GSBASE.
> * Note that, on SMP, the boot cpu uses init data section until
> * the per cpu areas are set up.
> */
> movl $MSR_GS_BASE,%ecx
> - movq $INIT_PER_CPU_VAR(fixed_percpu_data),%rax
> - cdq
> + xorl %eax, %eax
> + xorl %edx, %edx
... and here.
Uros.
> wrmsr
>
> mov %rsi, %rdi
> diff --git a/init/Kconfig b/init/Kconfig
> index c521e1421ad4..b374c0de5cfd 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1849,7 +1849,7 @@ config KALLSYMS_ALL
> config KALLSYMS_ABSOLUTE_PERCPU
> bool
> depends on KALLSYMS
> - default X86_64 && SMP
> + default n
>
> # end of the "standard kernel features (expert users)" menu
>
> --
> 2.47.0
>
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