Messages in this thread | | | Subject | Re: [PATCH v3 1/2] binfmt_flat: allow not offsetting data start | From | Greg Ungerer <> | Date | Fri, 16 Apr 2021 17:24:09 +1000 |
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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@wdc.com> >>> Acked-by: Palmer Dabbelt <palmerdabbelt@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; >>> } >>> } >>> >> > >
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