If the BSS section is empty, then the function should return.
If one does not check whether the BSS section is empty or not, then there is a
risk of writing 0s outside of BSS section (which may contain critical data).

Fixes: dac84b66cc9a ("xen: arm64: initial build + config changes, start of day code")
Signed-off-by: Ayan Kumar Halder <ayan.kumar.halder@amd.com>
---
Changes from :-

v1..v2 - New patch introduced in v3.

v3 - 1. Update the check in arm32 as well.
2. Drop the R-bs.

 xen/arch/arm/arm32/head.S | 3 +++
 xen/arch/arm/arm64/head.S | 2 ++
 2 files changed, 5 insertions(+)

diff --git a/xen/arch/arm/arm32/head.S b/xen/arch/arm/arm32/head.S
index a96d5d3503..4ff5c220bc 100644
--- a/xen/arch/arm/arm32/head.S
+++ b/xen/arch/arm/arm32/head.S
@@ -185,12 +185,15 @@ zero_bss:
         PRINT("- Zero BSS -\r\n")
         mov_w r0, __bss_start        /* r0 := vaddr(__bss_start) */
         mov_w r1, __bss_end          /* r1 := vaddr(__bss_end)   */
+        cmp   r1, r0
+        beq   skip_bss
 
         mov   r2, #0
 1:      str   r2, [r0], #4
         cmp   r0, r1
         blo   1b
 
+skip_bss:
         mov   pc, lr
 ENDPROC(zero_bss)
 
diff --git a/xen/arch/arm/arm64/head.S b/xen/arch/arm/arm64/head.S
index 14c3720d80..72c7b24498 100644
--- a/xen/arch/arm/arm64/head.S
+++ b/xen/arch/arm/arm64/head.S
@@ -346,6 +346,8 @@ FUNC_LOCAL(zero_bss)
         PRINT("- Zero BSS -\r\n")
         ldr   x0, =__bss_start       /* x0 := vaddr(__bss_start) */
         ldr   x1, =__bss_end         /* x1 := vaddr(__bss_end)   */
+        cmp   x1, x0
+        beq   skip_bss
 
 1:      str   xzr, [x0], #8
         cmp   x0, x1
-- 
2.25.1

Hi Ayan,

On 28/10/2024 12:45, Ayan Kumar Halder wrote:
> If the BSS section is empty, then the function should return.
> If one does not check whether the BSS section is empty or not, then there is a
> risk of writing 0s outside of BSS section (which may contain critical data).
> 
> Fixes: dac84b66cc9a ("xen: arm64: initial build + config changes, start of day code")
> Signed-off-by: Ayan Kumar Halder <ayan.kumar.halder@amd.com>

Reviewed-by: Julien Grall <jgrall@amazon.com>

Cheers,

-- 
Julien Grall

Hi Ayan,

> On 28 Oct 2024, at 12:45, Ayan Kumar Halder <ayan.kumar.halder@amd.com> wrote:
> 
> If the BSS section is empty, then the function should return.
> If one does not check whether the BSS section is empty or not, then there is a
> risk of writing 0s outside of BSS section (which may contain critical data).
> 
> Fixes: dac84b66cc9a ("xen: arm64: initial build + config changes, start of day code")
> Signed-off-by: Ayan Kumar Halder <ayan.kumar.halder@amd.com>
> —

Looks good to me

Reviewed-by: Luca Fancellu <luca.fancellu@arm.com>