[PATCH bpf-next 4/9] uprobes/x86: Don't WARN if an optimized probe can't be written

[Alexei Starovoitov]

From: Alexei Starovoitov <ast@xxxxxxxxxx>

__arch_uprobe_optimize() and set_orig_insn() do WARN_ON_ONCE() when
uprobe_write() fails, and user space can make it fail:
- -EINTR from get_user_pages_remote() when current is being killed.
- -ENOMEM when COW can't be broken, since memcg is at its limit.
- -EINVAL from __update_ref_ctr() when the process cleared the semaphore
  of SDT probe.

Kill the processes that call probed nop10. It warns in 8 of 8 runs:

  WARNING: arch/x86/kernel/uprobes.c:1186 at arch_uprobe_optimize+0x291/0x3a0
   uprobe_notify_resume+0x1c3/0x4c0
   exit_to_user_mode_loop+0x63/0x4e0
   exc_int3+0x1f3/0x250

The next patch calls set_orig_insn() from fork(). Kill the processes that
fork and keep mmap_lock busy, while a trace event on nop10 is enabled and
disabled. The kernel with the next two patches and without this one warns
in 3 of 3 runs. The whole series in 0 of 3:

  WARNING: arch/x86/kernel/uprobes.c:1161 at set_orig_insn+0x161/0x170
   register_one_mm+0x154/0x180
   uprobe_dup_mmap+0x61/0xb0
   dup_mmap+0x149/0x9f0

The callers handle the error. arch_uprobe_optimize() marks uprobe as not
optimizable and byte 0 stays a breakpoint. set_orig_insn() returns
the error. Drop both WARNs.

This depends on the previous patch. Without it WARN was the only sign of
the bytes that set_orig_insn() cannot restore. Hence no Fixes tag.

Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxx>
---
 arch/x86/kernel/uprobes.c | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kernel/uprobes.c b/arch/x86/kernel/uprobes.c
index 62bbe41c6466..33c35bf07801 100644
--- a/arch/x86/kernel/uprobes.c
+++ b/arch/x86/kernel/uprobes.c
@@ -1170,11 +1170,8 @@ int set_orig_insn(struct arch_uprobe *auprobe, struct vm_area_struct *vma,
 		int ret = is_optimized(vma->vm_mm, vaddr);
 		if (ret < 0)
 			return ret;
-		if (ret) {
-			ret = swbp_unoptimize(auprobe, vma, vaddr);
-			WARN_ON_ONCE(ret);
-			return ret;
-		}
+		if (ret)
+			return swbp_unoptimize(auprobe, vma, vaddr);
 	}
 	return uprobe_write_opcode(auprobe, vma, vaddr, *(uprobe_opcode_t *)&auprobe->insn,
 				   false /* is_register */);
@@ -1197,7 +1194,7 @@ static int __arch_uprobe_optimize(struct arch_uprobe *auprobe, struct mm_struct
 	if (IS_ERR(tramp))
 		return PTR_ERR(tramp);
 	ret = swbp_optimize(auprobe, vma, vaddr, tramp->vm_start);
-	if (WARN_ON_ONCE(ret) && new_mapping)
+	if (ret && new_mapping)
 		WARN_ON_ONCE(do_munmap(mm, tramp->vm_start, PAGE_SIZE, NULL));
 	return ret;
 }
-- 
2.55.0