[PATCH v3] MIPS: remove .bss to __rel_start overlay

[PATCH v3] MIPS: remove .bss to __rel_start overlay

[Oleksij Rempel]

.bss __rel_start (OVERLAY) was used to optimize RAM size used by
barebox. Since .bss and __rel_start overlap, we should clear bss only
after __rel_start was used. Following patch changed initialization
sequence to clear .bss before __rel_start:
1e5aef61fc6a444 ("MIPS: reloc: init bss and cpu")

This make relocation buffer unusable and broke relocation with different
symptoms reported by Antony Pavlov <antonynpavlov@gmail.com>:
  * iomem output has no information on sdram regions, so memtest is unusable;
  * pc = 0xa081232c, relocation does not work, barebox is located with 8M offset
    from start of RAM. The board has 256M and relocation routine
    should move barebox code much higher;
  * pc = 0xa081232c, so barebox code works from KSEG1 not from KSEG0 as MMU=y option implies.

There is a choice of moving .bss clear sequence after __rel_start or
remove this optimization. Since the use of this optimization is minimal
and danger to trap in to similar issue is still high, i prefer to remove
this optimization.

As result of this change, we need to fix calculation of relocation are
size:
relocate_code() calculates relocation area size as a sum (barebox_image_size +
bss_len). barebox_image_size and bss_len are calculated as (__image_end -
__image_start) and (__bss_stop - __bss_start) respectively. This doesn't take
into account relocation data placed between  __image_end and __bss_start.
However relocation preserves BSS position relative to image start, as if
relocation data is still there. This causes RAM overflow during BSS
initialization in main_entry(). This problem may be hidden due to the alignment
of the `relocaddr`.

Reported-by: Antony Pavlov <antonynpavlov@gmail.com>
Fixes: 1e5aef61fc6a444 ("MIPS: reloc: init bss and cpu")
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Peter Mamonov <pmamonov@gmail.com>
---
 arch/mips/lib/barebox.lds.S | 2 +-
 arch/mips/lib/reloc.c       | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/mips/lib/barebox.lds.S b/arch/mips/lib/barebox.lds.S
index 693a778980..c954df41f3 100644
--- a/arch/mips/lib/barebox.lds.S
+++ b/arch/mips/lib/barebox.lds.S
@@ -59,7 +59,7 @@ SECTIONS
 
 	_end = .;
 
-	.bss __rel_start (OVERLAY) : {
+	.bss : {
 		__bss_start = .;
 		*(.sbss.*)
 		*(.bss.*)
diff --git a/arch/mips/lib/reloc.c b/arch/mips/lib/reloc.c
index df9760e38c..4b0e252352 100644
--- a/arch/mips/lib/reloc.c
+++ b/arch/mips/lib/reloc.c
@@ -118,7 +118,7 @@ void relocate_code(void *fdt, u32 fdt_size, u32 ram_size)
 	memset(__bss_start, 0, bss_len);
 	cpu_probe();
 
-	length = barebox_image_size + bss_len;
+	length = __bss_stop - __image_start;
 	relocaddr = ALIGN_DOWN(ram_size - length, SZ_64K);
 	relocaddr = KSEG0ADDR(relocaddr);
 	new_stack = relocaddr - MALLOC_SIZE - 16;
-- 
2.25.0


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Re: [PATCH v3] MIPS: remove .bss to __rel_start overlay

[Peter Mamonov]

Probably "MIPS: don't overlay relocation data with BSS" sounds a bit more
human-like :)

Regards,
Peter

On Wed, Jan 29, 2020 at 06:27:37AM +0100, Oleksij Rempel wrote:
> .bss __rel_start (OVERLAY) was used to optimize RAM size used by
> barebox. Since .bss and __rel_start overlap, we should clear bss only
> after __rel_start was used. Following patch changed initialization
> sequence to clear .bss before __rel_start:
> 1e5aef61fc6a444 ("MIPS: reloc: init bss and cpu")
> 
> This make relocation buffer unusable and broke relocation with different
> symptoms reported by Antony Pavlov <antonynpavlov@gmail.com>:
>   * iomem output has no information on sdram regions, so memtest is unusable;
>   * pc = 0xa081232c, relocation does not work, barebox is located with 8M offset
>     from start of RAM. The board has 256M and relocation routine
>     should move barebox code much higher;
>   * pc = 0xa081232c, so barebox code works from KSEG1 not from KSEG0 as MMU=y option implies.
> 
> There is a choice of moving .bss clear sequence after __rel_start or
> remove this optimization. Since the use of this optimization is minimal
> and danger to trap in to similar issue is still high, i prefer to remove
> this optimization.
> 
> As result of this change, we need to fix calculation of relocation are
> size:
> relocate_code() calculates relocation area size as a sum (barebox_image_size +
> bss_len). barebox_image_size and bss_len are calculated as (__image_end -
> __image_start) and (__bss_stop - __bss_start) respectively. This doesn't take
> into account relocation data placed between  __image_end and __bss_start.
> However relocation preserves BSS position relative to image start, as if
> relocation data is still there. This causes RAM overflow during BSS
> initialization in main_entry(). This problem may be hidden due to the alignment
> of the `relocaddr`.
> 
> Reported-by: Antony Pavlov <antonynpavlov@gmail.com>
> Fixes: 1e5aef61fc6a444 ("MIPS: reloc: init bss and cpu")
> Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
> Signed-off-by: Peter Mamonov <pmamonov@gmail.com>
> ---
>  arch/mips/lib/barebox.lds.S | 2 +-
>  arch/mips/lib/reloc.c       | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/mips/lib/barebox.lds.S b/arch/mips/lib/barebox.lds.S
> index 693a778980..c954df41f3 100644
> --- a/arch/mips/lib/barebox.lds.S
> +++ b/arch/mips/lib/barebox.lds.S
> @@ -59,7 +59,7 @@ SECTIONS
>  
>  	_end = .;
>  
> -	.bss __rel_start (OVERLAY) : {
> +	.bss : {
>  		__bss_start = .;
>  		*(.sbss.*)
>  		*(.bss.*)
> diff --git a/arch/mips/lib/reloc.c b/arch/mips/lib/reloc.c
> index df9760e38c..4b0e252352 100644
> --- a/arch/mips/lib/reloc.c
> +++ b/arch/mips/lib/reloc.c
> @@ -118,7 +118,7 @@ void relocate_code(void *fdt, u32 fdt_size, u32 ram_size)
>  	memset(__bss_start, 0, bss_len);
>  	cpu_probe();
>  
> -	length = barebox_image_size + bss_len;
> +	length = __bss_stop - __image_start;
>  	relocaddr = ALIGN_DOWN(ram_size - length, SZ_64K);
>  	relocaddr = KSEG0ADDR(relocaddr);
>  	new_stack = relocaddr - MALLOC_SIZE - 16;
> -- 
> 2.25.0
> 

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