Re: [PATCH v2 2/2] riscv: support the elf-fdpic binfmt loader

[Greg Ungerer <gerg@kernel.org>]

On 13/7/23 01:12, Stefan O'Rear wrote:
> 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. 

True, not used by the ELF binaries themselves. But used by an ELF
interpreter to do the runtime relocations.


> 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.

This comment in the crt1.S code of uClibc made me think that a0 already had
a pre-defined use in the ABI:

     /* The entry point's job is to call __uClibc_main.  Per the ABI,
        a0 contains the address of a function to be passed to atexit.

But I didn't dig any further than that.

Regards
Greg


> -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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