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

[Greg Ungerer]

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 at wdc.com>
>>> Acked-by: Palmer Dabbelt <palmerdabbelt at google.com>
>>> ---
>>>    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.

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.

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.

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;
>>>    		}
>>>    	}
>>>
>>
> 
>