[PATCH v3 1/2] binfmt_flat: allow not offsetting data start
Damien Le Moal
Damien.LeMoal at wdc.com
Fri Apr 16 08:35:02 BST 2021
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 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.
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
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