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Date:   Wed, 12 Jul 2023 11:12:45 -0400
From:   "Stefan O'Rear" <sorear@fastmail.com>
To:     "Greg Ungerer" <gerg@kernel.org>, linux-riscv@lists.infradead.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Cc:     "Palmer Dabbelt" <palmer@dabbelt.com>, viro@zeniv.linux.org.uk,
        ebiederm@xmission.com, keescook@chromium.org, brauner@kernel.org
Subject: Re: [PATCH v2 2/2] riscv: support the elf-fdpic binfmt loader
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On Tue, Jul 11, 2023, at 9:07 AM, Greg Ungerer wrote:
> Add support for enabling and using the binfmt_elf_fdpic program loader
> on RISC-V platforms. The most important change is to setup registers
> during program load to pass the mapping addresses to the new process.
>
> One of the interesting features of the elf-fdpic loader is that it
> also allows appropriately compiled ELF format binaries to be loaded on
> nommu systems. Appropriate being those compiled with -pie.
>
> Signed-off-by: Greg Ungerer <gerg@kernel.org>
> ---
> v1->v2: rebase onto linux-6.5-rc1
>         increment PTRACE_GETFDPIC value to keep it unique
>
>  arch/riscv/include/asm/elf.h         | 11 ++++++++++-
>  arch/riscv/include/asm/mmu.h         |  4 ++++
>  arch/riscv/include/uapi/asm/ptrace.h |  5 +++++
>  fs/Kconfig.binfmt                    |  2 +-
>  4 files changed, 20 insertions(+), 2 deletions(-)
>
> diff --git a/arch/riscv/include/asm/elf.h b/arch/riscv/include/asm/elf.h
> index c24280774caf..c33fe923ef6d 100644
> --- a/arch/riscv/include/asm/elf.h
> +++ b/arch/riscv/include/asm/elf.h
> @@ -41,6 +41,7 @@ extern bool compat_elf_check_arch(Elf32_Ehdr *hdr);
>  #define compat_elf_check_arch	compat_elf_check_arch
> 
>  #define CORE_DUMP_USE_REGSET
> +#define ELF_FDPIC_CORE_EFLAGS	0
>  #define ELF_EXEC_PAGESIZE	(PAGE_SIZE)
> 
>  /*
> @@ -69,6 +70,13 @@ extern bool compat_elf_check_arch(Elf32_Ehdr *hdr);
>  #define ELF_HWCAP	riscv_get_elf_hwcap()
>  extern unsigned long elf_hwcap;
> 
> +#define ELF_FDPIC_PLAT_INIT(_r, _exec_map_addr, _interp_map_addr, 
> dynamic_addr) \
> +	do { \
> +		(_r)->a1 = _exec_map_addr; \
> +		(_r)->a2 = _interp_map_addr; \
> +		(_r)->a3 = dynamic_addr; \
> +	} while (0)
> +

This should probably be left empty for now; it will be defined by the
ELF FDPIC ABI when that is done, and shouldn't be used by normal ELF
binaries.  I'd ask if there's a reason it starts at a1 instead of a0,
but it seems idiosyncratic on all arches that have full FDPIC support.

-s

>  /*
>   * This yields a string that ld.so will use to load implementation
>   * specific libraries for optimization.  This is more specific in
> @@ -78,7 +86,6 @@ extern unsigned long elf_hwcap;
> 
>  #define COMPAT_ELF_PLATFORM	(NULL)
> 
> -#ifdef CONFIG_MMU
>  #define ARCH_DLINFO						\
>  do {								\
>  	/*							\
> @@ -115,6 +122,8 @@ do {								\
>  	else							 \
>  		NEW_AUX_ENT(AT_IGNORE, 0);			 \
>  } while (0)
> +
> +#ifdef CONFIG_MMU
>  #define ARCH_HAS_SETUP_ADDITIONAL_PAGES
>  struct linux_binprm;
>  extern int arch_setup_additional_pages(struct linux_binprm *bprm,
> diff --git a/arch/riscv/include/asm/mmu.h b/arch/riscv/include/asm/mmu.h
> index 0099dc116168..355504b37f8e 100644
> --- a/arch/riscv/include/asm/mmu.h
> +++ b/arch/riscv/include/asm/mmu.h
> @@ -20,6 +20,10 @@ typedef struct {
>  	/* A local icache flush is needed before user execution can resume. */
>  	cpumask_t icache_stale_mask;
>  #endif
> +#ifdef CONFIG_BINFMT_ELF_FDPIC
> +	unsigned long exec_fdpic_loadmap;
> +	unsigned long interp_fdpic_loadmap;
> +#endif
>  } mm_context_t;
> 
>  void __init create_pgd_mapping(pgd_t *pgdp, uintptr_t va, phys_addr_t 
> pa,
> diff --git a/arch/riscv/include/uapi/asm/ptrace.h 
> b/arch/riscv/include/uapi/asm/ptrace.h
> index e17c550986a6..30f6d6537adc 100644
> --- a/arch/riscv/include/uapi/asm/ptrace.h
> +++ b/arch/riscv/include/uapi/asm/ptrace.h
> @@ -10,6 +10,11 @@
> 
>  #include <linux/types.h>
> 
> +#define PTRACE_GETFDPIC		33
> +
> +#define PTRACE_GETFDPIC_EXEC	0
> +#define PTRACE_GETFDPIC_INTERP	1
> +
>  /*
>   * User-mode register state for core dumps, ptrace, sigcontext
>   *
> diff --git a/fs/Kconfig.binfmt b/fs/Kconfig.binfmt
> index 93539aac0e5b..f5693164ca9a 100644
> --- a/fs/Kconfig.binfmt
> +++ b/fs/Kconfig.binfmt
> @@ -58,7 +58,7 @@ config ARCH_USE_GNU_PROPERTY
>  config BINFMT_ELF_FDPIC
>  	bool "Kernel support for FDPIC ELF binaries"
>  	default y if !BINFMT_ELF
> -	depends on ARM || ((M68K || SUPERH || XTENSA) && !MMU)
> +	depends on ARM || ((M68K || RISCV || SUPERH || XTENSA) && !MMU)
>  	select ELFCORE
>  	help
>  	  ELF FDPIC binaries are based on ELF, but allow the individual load
> -- 
> 2.25.1
>
>
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