Re: [PATCH v2] binfmt_flat: do not stop relocating GOT entries prematurely on riscv

Damien Le Moal
2022-04-14T23:51:37Z

On 4/14/22 18:10, Niklas Cassel wrote:
> bFLT binaries are usually created using elf2flt.
>
> The linker script used by elf2flt has defined the .data section like the
> following for the last 19 years:
>
> .data : {
> _sdata = . ;
> __data_start = . ;
> data_start = . ;
> *(.got.plt)
> *(.got)
> FILL(0) ;
> . = ALIGN(0x20) ;
> LONG(-1)
> . = ALIGN(0x20) ;
> ...
> }
>
> It places the .got.plt input section before the .got input section.
> The same is true for the default linker script (ld --verbose) on most
> architectures except x86/x86-64.
>
> The binfmt_flat loader should relocate all GOT entries until it encounters
> a -1 (the LONG(-1) in the linker script).
>
> The problem is that the .got.plt input section starts with a GOTPLT header
> (which has size 16 bytes on elf64-riscv and 8 bytes on elf32-riscv), where
> the first word is set to -1. See the binutils implementation for riscv [1].
>
> This causes the binfmt_flat loader to stop relocating GOT entries
> prematurely and thus causes the application to crash when running.
>
> Fix this by skipping the whole GOTPLT header, since the whole GOTPLT header
> is reserved for the dynamic linker.
>
> The GOTPLT header will only be skipped for bFLT binaries with flag
> FLAT_FLAG_GOTPIC set. This flag is unconditionally set by elf2flt if the
> supplied ELF binary has the symbol _GLOBAL_OFFSET_TABLE_ defined.
> ELF binaries without a .got input section should thus remain unaffected.
>
> Tested on RISC-V Canaan Kendryte K210 and RISC-V QEMU nommu_virt_defconfig.
>
> [1] https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=bfd/elfnn-riscv.c;hb=binutils-2_38#l3275
>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Niklas Cassel <niklas.cassel@wdc.com>
> ---
> Changes since v1:
> -Incorporated review comments from Eric Biederman.
>
> RISC-V elf2flt patches are still not merged, they can be found here:
> https://github.com/floatious/elf2flt/tree/riscv
>
> buildroot branch for k210 nommu (including this patch and elf2flt patches):
> https://github.com/floatious/buildroot/tree/k210-v14
>
> fs/binfmt_flat.c | 27 ++++++++++++++++++++++++++-
> 1 file changed, 26 insertions(+), 1 deletion(-)
>
> diff --git a/fs/binfmt_flat.c b/fs/binfmt_flat.c
> index 626898150011..e5e2a03b39c1 100644
> --- a/fs/binfmt_flat.c
> +++ b/fs/binfmt_flat.c
> @@ -440,6 +440,30 @@ static void old_reloc(unsigned long rl)
>
> /****************************************************************************/
>
> +static inline u32 __user *skip_got_header(u32 __user *rp)
> +{
> + if (IS_ENABLED(CONFIG_RISCV)) {
> + /*
> + * RISC-V has a 16 byte GOT PLT header for elf64-riscv
> + * and 8 byte GOT PLT header for elf32-riscv.
> + * Skip the whole GOT PLT header, since it is reserved
> + * for the dynamic linker (ld.so).
> + */
> + u32 rp_val0, rp_val1;
> +
> + if (get_user(rp_val0, rp))
> + return rp;
> + if (get_user(rp_val1, rp + 1))
> + return rp;
> +
> + if (rp_val0 == 0xffffffff && rp_val1 == 0xffffffff)
> + rp += 4;
> + else if (rp_val0 == 0xffffffff)
> + rp += 2;

This looks good to me. But thinking more about it, do we really need to
check what the content of the header is ? Why not simply replace this
entire hunk with:

return rp + sizeof(unsigned long) * 2;

to ignore the 16B (or 8B for 32-bits arch) header regardless of what the
header word values are ? Are there any case where the header is *not*
present ?

> + }
> + return rp;
> +}
> +
> static int load_flat_file(struct linux_binprm *bprm,
> struct lib_info *libinfo, int id, unsigned long *extra_stack)
> {
> @@ -789,7 +813,8 @@ static int load_flat_file(struct linux_binprm *bprm,
> * image.
> */
> if (flags & FLAT_FLAG_GOTPIC) {
> - for (rp = (u32 __user *)datapos; ; rp++) {
> + rp = skip_got_header((u32 * __user) datapos);
> + for (; ; rp++) {
> u32 addr, rp_val;
> if (get_user(rp_val, rp))
> return -EFAULT;

Regardless of the above nit, feel free to add:

Reviewed-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>

--
Damien Le Moal
Western Digital Research