Re: [PATCH v3 1/2] binfmt_flat: allow not offsetting data start

[Damien Le Moal]

On 2021/04/16 16:24, Greg Ungerer wrote:
> 
> On 16/4/21 9:22 am, Damien Le Moal wrote:
>> On 2021/04/15 23:04, Greg Ungerer wrote:
>>> Hi Damien,
>>>
>>> On 15/4/21 4:15 pm, Damien Le Moal wrote:
>>>> Commit 2217b9826246 ("binfmt_flat: revert "binfmt_flat: don't offset
>>>> the data start"") restored offsetting the start of the data section by
>>>> a number of words defined by MAX_SHARED_LIBS. As a result, since
>>>> MAX_SHARED_LIBS is never 0, a gap between the text and data sections
>>>> always exists. For architectures which cannot support a such gap
>>>> between the text and data sections (e.g. riscv nommu), flat binary
>>>> programs cannot be executed.
>>>>
>>>> To allow an architecture to request contiguous text and data sections,
>>>> introduce the config option CONFIG_BINFMT_FLAT_NO_TEXT_DATA_GAP.
>>>> Using this new option, the macro DATA_GAP_WORDS is conditionally
>>>> defined in binfmt_flat.c to MAX_SHARED_LIBS for architectures
>>>> tolerating the text-to-data gap (CONFIG_BINFMT_FLAT_NO_TEXT_DATA_GAP
>>>> disabled case) and to 0 when CONFIG_BINFMT_FLAT_NO_TEXT_DATA_GAP is
>>>> enabled. DATA_GAP_WORDS is used in load_flat_file() to calculate the
>>>> data section length and start position.
>>>>
>>>> An architecture enabling CONFIG_BINFMT_FLAT_NO_TEXT_DATA_GAP also
>>>> prevents the use of the separate text/data load case (when the flat file
>>>> header flags FLAT_FLAG_RAM and FLAT_FLAG_GZIP are not set with NOMMU
>>>> kernels) and forces the use of a single RAM region for loading
>>>> (equivalent to FLAT_FLAG_RAM being set).
>>>
>>> So is it the case that a flat format file on RISC-V will never have
>>> relocations?
>>
>> No, it does have relocations. But there is no entry for the global pointer
>> (__global_pointer$) location. This is because the loading of that value in the
>> gp register in the C-library crt1.S is done using a PC-relative instruction. The
>> value for it is resolved at compile time and does not get a relocation table
>> entry. Other functions calls and symbol references do have relocation table
>> entries, so the binary can be loaded anywhere. The missing relocation for the
>> global pointer mandates that text and data be loaded at the same positions
>> relative to each other that the linker file defines. Otherwise, loading of
>> __global_pointer$ into the gp register (first thing that C libraries crt1.S do)
>> result in a garbage value being loaded.
>>
>> I tried some tricks with the linker file and changing uclibc crt1.S to have the
>> gp loading done using a symbol address instead of a PC-relative offset. I could
>> then see a relocation table entry for that symbol. That still did not work as I
>> was probably doing something wrong. Anyway, such solution requires changing a
>> lot of things in C libraries loading assembler that is common between NOMMU and
>> MMU code. Changing it would break MMU enabled programs.
>>
>>
>>>> Signed-off-by: Damien Le Moal <damien.lemoal@xxxxxxx>
>>>> Acked-by: Palmer Dabbelt <palmerdabbelt@xxxxxxxxxx>
>>>> ---
>>>>    fs/Kconfig.binfmt |  3 +++
>>>>    fs/binfmt_flat.c  | 21 +++++++++++++++------
>>>>    2 files changed, 18 insertions(+), 6 deletions(-)
>>>>
>>>> diff --git a/fs/Kconfig.binfmt b/fs/Kconfig.binfmt
>>>> index c6f1c8c1934e..c6df931d5d45 100644
>>>> --- a/fs/Kconfig.binfmt
>>>> +++ b/fs/Kconfig.binfmt
>>>> @@ -112,6 +112,9 @@ config BINFMT_FLAT_ARGVP_ENVP_ON_STACK
>>>>    config BINFMT_FLAT_OLD_ALWAYS_RAM
>>>>    	bool
>>>>    
>>>> +config BINFMT_FLAT_NO_TEXT_DATA_GAP
>>>> +	bool
>>>> +
>>>>    config BINFMT_FLAT_OLD
>>>>    	bool "Enable support for very old legacy flat binaries"
>>>>    	depends on BINFMT_FLAT
>>>> diff --git a/fs/binfmt_flat.c b/fs/binfmt_flat.c
>>>> index b9c658e0548e..2be29bb964b8 100644
>>>> --- a/fs/binfmt_flat.c
>>>> +++ b/fs/binfmt_flat.c
>>>> @@ -74,6 +74,12 @@
>>>>    #define	MAX_SHARED_LIBS			(1)
>>>>    #endif
>>>>    
>>>> +#ifdef CONFIG_BINFMT_FLAT_NO_TEXT_DATA_GAP
>>>> +#define DATA_GAP_WORDS			(0)
>>>> +#else
>>>> +#define DATA_GAP_WORDS			(MAX_SHARED_LIBS)
>>>> +#endif
>>>> +>   struct lib_info {
>>>>    	struct {
>>>>    		unsigned long start_code;		/* Start of text segment */
>>>> @@ -559,7 +565,10 @@ static int load_flat_file(struct linux_binprm *bprm,
>>>>    	 * case,  and then the fully copied to RAM case which lumps
>>>>    	 * it all together.
>>>>    	 */
>>>> -	if (!IS_ENABLED(CONFIG_MMU) && !(flags & (FLAT_FLAG_RAM|FLAT_FLAG_GZIP))) {
>>>> +	if (!IS_ENABLED(CONFIG_MMU) &&
>>>> +	    !IS_ENABLED(CONFIG_BINFMT_FLAT_NO_TEXT_DATA_GAP) &&
>>>
>>> If RISC-V flat format files must always be loaded to RAM then why don't
>>> they set the FLAT_FLAG_RAM when compiled/generated?
>>
>> That is done. The patch I have for elf2flt sets it. Coding it like this here is
>> I think safer (whatever the userspace toolchain did, the kernel assumes
>> FLAT_FLAG_RAM). And it also has the nice side effect to suppress the first part
>> of the if () in the final binary. Smaller code size :)
> 
> My concern here is that CONFIG_BINFMT_FLAT_NO_TEXT_DATA_GA being
> enabled doesn't just in itself mean you need to force a RAM load.
> It is just in the RISC-V case it currently does.

Good point.

> 
> And it may change in the future. The considerable RAM savings
> you get from supporting a separate data segment to code segment
> means there is motivation to create tooling and code generation
> to support it.

Totally agree here. And I did try hard to get it to work...

> 
> I don't feel that strongly about it, but this code is obtuse enough already.
> No need to make it worse if we don't have too.

I see your point. I will remove that
!IS_ENABLED(CONFIG_BINFMT_FLAT_NO_TEXT_DATA_GAP) from the top if() condition.
Without it, fixing the toolchain etc will indeed not require patching again the
kernel. Sending a v4.

> 
> Regards
> Greg
> 
> 
> 
>>>> +	    !(flags & (FLAT_FLAG_RAM|FLAT_FLAG_GZIP))) {
>>>> +
>>>>    		/*
>>>>    		 * this should give us a ROM ptr,  but if it doesn't we don't
>>>>    		 * really care
>>>> @@ -576,7 +585,7 @@ static int load_flat_file(struct linux_binprm *bprm,
>>>>    			goto err;
>>>>    		}
>>>>    
>>>> -		len = data_len + extra + MAX_SHARED_LIBS * sizeof(unsigned long);
>>>> +		len = data_len + extra + DATA_GAP_WORDS * sizeof(unsigned long);
>>>>    		len = PAGE_ALIGN(len);
>>>>    		realdatastart = vm_mmap(NULL, 0, len,
>>>>    			PROT_READ|PROT_WRITE|PROT_EXEC, MAP_PRIVATE, 0);
>>>> @@ -591,7 +600,7 @@ static int load_flat_file(struct linux_binprm *bprm,
>>>>    			goto err;
>>>>    		}
>>>>    		datapos = ALIGN(realdatastart +
>>>> -				MAX_SHARED_LIBS * sizeof(unsigned long),
>>>> +				DATA_GAP_WORDS * sizeof(unsigned long),
>>>>    				FLAT_DATA_ALIGN);
>>>>    
>>>>    		pr_debug("Allocated data+bss+stack (%u bytes): %lx\n",
>>>> @@ -622,7 +631,7 @@ static int load_flat_file(struct linux_binprm *bprm,
>>>>    		memp_size = len;
>>>>    	} else {
>>>>    
>>>> -		len = text_len + data_len + extra + MAX_SHARED_LIBS * sizeof(u32);
>>>> +		len = text_len + data_len + extra + DATA_GAP_WORDS * sizeof(u32);
>>>>    		len = PAGE_ALIGN(len);
>>>>    		textpos = vm_mmap(NULL, 0, len,
>>>>    			PROT_READ | PROT_EXEC | PROT_WRITE, MAP_PRIVATE, 0);
>>>> @@ -638,7 +647,7 @@ static int load_flat_file(struct linux_binprm *bprm,
>>>>    
>>>>    		realdatastart = textpos + ntohl(hdr->data_start);
>>>>    		datapos = ALIGN(realdatastart +
>>>> -				MAX_SHARED_LIBS * sizeof(u32),
>>>> +				DATA_GAP_WORDS * sizeof(u32),
>>>>    				FLAT_DATA_ALIGN);
>>>>    
>>>>    		reloc = (__be32 __user *)
>>>> @@ -714,7 +723,7 @@ static int load_flat_file(struct linux_binprm *bprm,
>>>>    			ret = result;
>>>>    			pr_err("Unable to read code+data+bss, errno %d\n", ret);
>>>>    			vm_munmap(textpos, text_len + data_len + extra +
>>>> -				MAX_SHARED_LIBS * sizeof(u32));
>>>> +				  DATA_GAP_WORDS * sizeof(u32));
>>>>    			goto err;
>>>>    		}
>>>>    	}
>>>>
>>>
>>
>>
> 


-- 
Damien Le Moal
Western Digital Research