Re: [BUG] x86: 32-bit uprobes are broken

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Jan 07, 2026 at 03:32:53PM +0100, Oleg Nesterov wrote:
> Paulo reported that uprobing the 32-bit tasks is broken.
> 
> This script
> 
> 	#!/usr/bin/bash
> 
> 	echo 0 > /proc/sys/kernel/randomize_va_space
> 
> 	echo 'void main(void) {}' > TEST.c
> 
> 	# -fcf-protection to ensure that the 1st endbr32 insn can't be eulated
> 	gcc -m32 -fcf-protection=branch TEST.c -o test
> 
> 	bpftrace -e 'uprobe:./test:main {}' -c ./test
> 
> "hangs", the probed ./test task enters the endless loop.
> 
> This patch
> 
> 	--- a/kernel/events/uprobes.c
> 	+++ b/kernel/events/uprobes.c
> 	@@ -1710,8 +1710,11 @@ static int xol_add_vma(struct mm_struct *mm, struct xol_area *area)
> 	 
> 		if (!area->vaddr) {
> 			/* Try to map as high as possible, this is only a hint. */
> 	+		if (test_thread_flag(TIF_ADDR32))
> 	+			current_thread_info()->status |= TS_COMPAT;
> 			area->vaddr = get_unmapped_area(NULL, TASK_SIZE - PAGE_SIZE,
> 							PAGE_SIZE, 0, 0);
> 	+		current_thread_info()->status &= ~TS_COMPAT;
> 			if (IS_ERR_VALUE(area->vaddr)) {
> 				ret = area->vaddr;
> 				goto fail;

Urgh. Also I'm a little bit confused; set_personality_ia32(), which is
what sets TIF_ADDR32 (afaict) would also set TS_COMPAT, no?

> or this one
> 
> 	--- a/arch/x86/kernel/sys_x86_64.c
> 	+++ b/arch/x86/kernel/sys_x86_64.c
> 	@@ -205,6 +205,8 @@ arch_get_unmapped_area_topdown(struct file *filp, unsigned long addr0,
> 			info.low_limit = PAGE_SIZE;
> 	 
> 		info.high_limit = get_mmap_base(0);
> 	+	if (test_thread_flag(TIF_ADDR32))
> 	+		info.high_limit = current->mm->mmap_compat_base;
> 		if (!(filp && is_file_hugepages(filp))) {
> 			info.start_gap = stack_guard_placement(vm_flags);
> 			info.align_offset = pgoff << PAGE_SHIFT;

This one would be broken if there is ever a case where
get_unmapped_area() is called for mm != current->mm.

Ah, but get_mmap_base() seems to already rely on that. What a mess :/

Why can't get_mmap_base() rely on TIF_ADDR32 rather than TS_COMPAT? What
funny games are being played here? Mixed mode mm where some threads are
32bit and others are not?

> "fixes" the problem.
> 
> -----------------------------------------------------------------------------------------
> The problem is that with randomize_va_space == 0 get_unmapped_area(TASK_SIZE - PAGE_SIZE)
> called by xol_add_vma() can't just return the "addr == TASK_SIZE - PAGE_SIZE" hint, this
> addr is used by the stack vma.
> 
> arch_get_unmapped_area/arch_get_unmapped_area_topdown() doesn't take TIF_ADDR32 into
> account and in_32bit_syscall() is false, this leads to info.high_limit > TASK_SIZE.
> get_area() happily returns the "high" address and then get_unmapped_area() returns
> ENOMEM after the
> 
> 	if (addr > TASK_SIZE - len)
> 		return -ENOMEM;
> 
> check; TASK_SIZE checks TIF_ADDR32.
> 
> handle_swbp() doesn't report this failure (probably it should) and silently restarts
> the probed insn. Endless loop.
> 
> -----------------------------------------------------------------------------------------
> I am considering the patch which adds something like
> 
> 	// x86 version will use the TS_COMPAT hack
> 	unsigned long __weak arch_uprobe_area(void)
> 	{
> 		return get_unmapped_area(NULL, TASK_SIZE - PAGE_SIZE, PAGE_SIZE, 0, 0);
> 	}
> 
> but perhaps there is a better solution? perhaps it makes more sense to change
> arch/x86/kernel/sys_x86_64.c? What is the point of ignoring TIF_ADDR32 if the
> high adress will be nacked by the "if (addr > TASK_SIZE - len)" check anyway?

To me it seems a bit weird that get_unmapped_area() is so disconnected
from mm_struct.