mt7620: workaround jal imm26 and redundant PAGE_ALIGN

On MT7620-class platforms (CONFIG_NET_RALINK_MT7620) we observe sporadic
wrong-jump-targets, kernel oopses, hanging, corrupted backtraces or even
"half-written" instructions when the compiler emits a direct 'jal imm26'
call.
This is triggered in:
  - the small random helpers inside get_random_u32_below(), and
  - the blkcg_maybe_throttle_current() call in resume_user_mode_work().

This patch forces those two call sites to use an indirect call via
a volatile function pointer (load into register + jalr) when building
for MT7620, avoiding embedding a 26-bit immediate jump target.

Additionally, on MT7620 builds the exec path in fs/exec.c is modified:
  - skip arch_align_stack() + PAGE_ALIGN() in setup_arg_pages()
    because the micro-randomization (< PAGE_SIZE) implemented by many
    ports (including MT7620) is negated immediately by PAGE_ALIGN().
    Skipping the redundant PAGE_ALIGN() reduces exposure to the
    problematic code pattern.

These changes are targeted workarounds for MT7620; behavioral logic is unchanged.

Signed-off-by: Mieczyslaw Nalewaj <namiltd@yahoo.com>
Link: https://github.com/openwrt/openwrt/pull/20553
Signed-off-by: Robert Marko <robimarko@gmail.com>

target/linux/ramips/patches-6.12/330-mt7620-avoid-direct-jal-in-random-helper.patch

@@ -0,0 +1,72 @@
+From: Mieczyslaw Nalewaj <namiltd@yahoo.com>
+Date: Sun, 26 Oct 2025 10:36:02 +0100
+Subject: [PATCH] mt7620: avoid direct jal imm26 in random helper
+
+This patch forces an indirect call (via a function pointer) on the mt7620
+platform when calling the small random helpers used by the constant-folding
+fast path of get_random_u32_below(). That avoids embedding a 26-bit immediate
+jump target in a direct `jal` instruction, which on affected platforms can
+lead to sporadic incorrect jump targets, intermittent oopses/crashes/hangs,
+corrupted backtraces, or "half-written" instructions under some circumstances.
+
+The change is targetted and conservative: it only alters code generation on
+CONFIG_NET_RALINK_MT7620 builds and does not change the algorithmic behaviour
+of get_random_u32_below().
+
+Signed-off-by: Mieczyslaw Nalewaj <namiltd@yahoo.com>
+---
+ a/include/linux/random.h | 36 ++++++++++++++++++++++++++++++++++++----
+ 1 file changed, 32 insertions(+), 4 deletions(-)
+
+--- a/include/linux/random.h
++++ b/include/linux/random.h
+@@ -73,17 +73,45 @@ static inline u32 get_random_u32_below(u
+ 	BUILD_BUG_ON_MSG(!ceil, "get_random_u32_below() must take ceil > 0");
+ 	if (ceil <= 1)
+ 		return 0;
+-	for (;;) {
+-		if (ceil <= 1U << 8) {
++	if (ceil <= 1U << 8) {
++#ifdef CONFIG_NET_RALINK_MT7620
++		typedef u8 (*getrnd_t)(void);
++		volatile getrnd_t rnd_fn = (getrnd_t)get_random_u8;
++#endif
++		for (;;) {
++#ifdef CONFIG_NET_RALINK_MT7620
++			u32 mult = ceil * rnd_fn();
++#else
+ 			u32 mult = ceil * get_random_u8();
++#endif
+ 			if (likely(is_power_of_2(ceil) || (u8)mult >= (1U << 8) % ceil))
+ 				return mult >> 8;
+-		} else if (ceil <= 1U << 16) {
++		}
++	} else if (ceil <= 1U << 16) {
++#ifdef CONFIG_NET_RALINK_MT7620
++		typedef u16 (*getrnd_t)(void);
++		volatile getrnd_t rnd_fn = (getrnd_t)get_random_u16;
++#endif
++		for (;;) {
++#ifdef CONFIG_NET_RALINK_MT7620
++			u32 mult = ceil * rnd_fn();
++#else
+ 			u32 mult = ceil * get_random_u16();
++#endif
+ 			if (likely(is_power_of_2(ceil) || (u16)mult >= (1U << 16) % ceil))
+ 				return mult >> 16;
+-		} else {
++		}
++	} else {
++#ifdef CONFIG_NET_RALINK_MT7620
++		typedef u32 (*getrnd_t)(void);
++		volatile getrnd_t rnd_fn = (getrnd_t)get_random_u32;
++#endif
++		for (;;) {
++#ifdef CONFIG_NET_RALINK_MT7620
++			u64 mult = (u64)ceil * rnd_fn();
++#else
+ 			u64 mult = (u64)ceil * get_random_u32();
++#endif
+ 			if (likely(is_power_of_2(ceil) || (u32)mult >= -ceil % ceil))
+ 				return mult >> 32;
+ 		}

target/linux/ramips/patches-6.12/331-mt7620-avoid-direct-jal-in-resume_user_mode_work.patch

@@ -0,0 +1,35 @@
+From: Mieczyslaw Nalewaj <namiltd@yahoo.com>
+Date: Tue, 28 Oct 2025 22:50:41 +0100
+Subject: [PATCH] mt7620: avoid direct jal imm26 in resume_user_mode_work
+
+This patch forces an indirect call (via a function pointer) on the mt7620
+platform when invoking blkcg_maybe_throttle_current() from
+resume_user_mode_work(). Avoiding a direct `jal` with a 26-bit immediate
+jump target prevents sporadic incorrect jump targets / intermittent oopses,
+crashes or hanging behavior that can appear on affected platforms.
+
+The change is targeted: it only alters the call site when CONFIG_NET_RALINK_MT7620
+is enabled and does not change the functional behaviour of resume_user_mode_work().
+
+Signed-off-by: Mieczyslaw Nalewaj <namiltd@yahoo.com>
+
+---
+ a/include/linux/resume_user_mode.h | 6 +++++-
+ 1 file changed, 5 insertions(+), 1 deletion(-)
+
+--- a/include/linux/resume_user_mode.h
++++ b/include/linux/resume_user_mode.h
+@@ -57,7 +57,13 @@ static inline void resume_user_mode_work
+ #endif
+ 
+ 	mem_cgroup_handle_over_high(GFP_KERNEL);
++#ifdef CONFIG_NET_RALINK_MT7620
++	typedef void (*bmtc_t)(void);
++	volatile bmtc_t bmtc_fn = (bmtc_t)blkcg_maybe_throttle_current;
++	bmtc_fn();
++#else
+ 	blkcg_maybe_throttle_current();
++#endif
+ 
+ 	rseq_handle_notify_resume(NULL, regs);
+ }

target/linux/ramips/patches-6.12/332-mt7620-conditional-stack-align.patch

@@ -0,0 +1,30 @@
+From: Mieczyslaw Nalewaj <namiltd@yahoo.com>
+Date: Sun, 26 Oct 2025 10:36:02 +0100
+Subject: [PATCH] mt7620: conditional stack align
+
+This patch avoids applying arch_align_stack() and PAGE_ALIGN() in the exec
+path on CONFIG_NET_RALINK_MT7620 builds. Many ports (including mt7620)
+implement only micro-randomization inside arch_align_stack() (random offset
+< PAGE_SIZE and then align-down to small alignment). Callers that immediately
+apply PAGE_ALIGN() will round that micro-offset back to the original page
+boundary, so invoking arch_align_stack() then PAGE_ALIGN() is pointless and
+can be avoided on the affected platform.
+
+Signed-off-by: Mieczyslaw Nalewaj <namiltd@yahoo.com>
+---
+ a/fs/exec.c | 2 ++
+ 1 file changed, 2 insertions(+)
+
+--- a/fs/exec.c
++++ b/fs/exec.c
+@@ -750,8 +750,10 @@ int setup_arg_pages(struct linux_binprm
+ 	mm->arg_start = bprm->p - stack_shift;
+ 	bprm->p = vma->vm_end - stack_shift;
+ #else
++#ifndef CONFIG_NET_RALINK_MT7620
+ 	stack_top = arch_align_stack(stack_top);
+ 	stack_top = PAGE_ALIGN(stack_top);
++#endif
+ 
+ 	if (unlikely(stack_top < mmap_min_addr) ||
+ 	    unlikely(vma->vm_end - vma->vm_start >= stack_top - mmap_min_addr))