Re: [PATCH v4 1/2] binfmt_flat: allow not offsetting data start

[Greg Ungerer]

On 17/4/21 11:10 am, 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 no data start offset to allow for
> contiguous text and data sections for binaries flagged with
> FLAT_FLAG_RAM, introduce the new config option
> CONFIG_BINFMT_FLAT_NO_DATA_START_OFFSET. Using this new option, the
> macro DATA_START_OFFSET_WORDS is conditionally defined in binfmt_flat.c
> to MAX_SHARED_LIBS for architectures tolerating or needing the data
> start offset (CONFIG_BINFMT_FLAT_NO_DATA_START_OFFSET disabled case)
> and to 0 when CONFIG_BINFMT_FLAT_NO_DATA_START_OFFSET is enabled.
> DATA_START_OFFSET_WORDS is used in load_flat_file() to calculate the
> data section length and start position.
>
> Signed-off-by: Damien Le Moal <damien.lemoal@xxxxxxx>
> ---
>   fs/Kconfig.binfmt |  3 +++
>   fs/binfmt_flat.c  | 19 ++++++++++++++-----
>   2 files changed, 17 insertions(+), 5 deletions(-)
>
> diff --git a/fs/Kconfig.binfmt b/fs/Kconfig.binfmt
> index c6f1c8c1934e..06fb7a93a1bd 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_DATA_START_OFFSET
> +	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..1dc68dfba3e0 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_DATA_START_OFFSET
> +#define DATA_START_OFFSET_WORDS		(0)
> +#else
> +#define DATA_START_OFFSET_WORDS		(MAX_SHARED_LIBS)
> +#endif
> +
>   struct lib_info {
>   	struct {
>   		unsigned long start_code;		/* Start of text segment */
> @@ -560,6 +566,7 @@ static int load_flat_file(struct linux_binprm *bprm,
>   	 * it all together.
>   	 */
>   	if (!IS_ENABLED(CONFIG_MMU) && !(flags & (FLAT_FLAG_RAM|FLAT_FLAG_GZIP))) {
> +

Random white space change...
Don't worry about re-spinning though, I will just edit this chunk out.


>   		/*
>   		 * this should give us a ROM ptr,  but if it doesn't we don't
>   		 * really care
> @@ -576,7 +583,8 @@ 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_START_OFFSET_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 +599,7 @@ static int load_flat_file(struct linux_binprm *bprm,
>   			goto err;
>   		}
>   		datapos = ALIGN(realdatastart +
> -				MAX_SHARED_LIBS * sizeof(unsigned long),
> +				DATA_START_OFFSET_WORDS * sizeof(unsigned long),
>   				FLAT_DATA_ALIGN);
>   
>   		pr_debug("Allocated data+bss+stack (%u bytes): %lx\n",
> @@ -622,7 +630,8 @@ 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_START_OFFSET_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_START_OFFSET_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_START_OFFSET_WORDS * sizeof(u32));
>   			goto err;
>   		}
>   	}

Thanks, otherwise looks good.

Acked-by: Greg Ungerer <gerg@xxxxxxxxxxxxxx>

I will push this into my m68knommu tree, for-next branch.
I just carry the flat format changes in that tree now to make my life easier.

Regards
Greg