RISC-V uprobe bug (Was: Re: WARNING: CPU: 3 PID: 261 at kernel/bpf/memalloc.c:342)

[Nam Cao]

On Sun, Aug 27, 2023 at 10:11:25AM +0200, Björn Töpel wrote:
> The default implementation of is_trap_insn() which RISC-V is using calls
> is_swbp_insn(), which is doing what your patch does. Your patch does not
> address the issue.

is_swbp_insn() does this:

        #ifdef CONFIG_RISCV_ISA_C
                return (*insn & 0xffff) == UPROBE_SWBP_INSN;
        #else
                return *insn == UPROBE_SWBP_INSN;
        #endif

...so it doesn't even check for 32-bit ebreak if C extension is on. My patch
is not the same.

But okay, if it doesn't solve the problem, then I must be wrong somewhere.
 
> We're taking an ebreak trap from kernel space. In this case we should
> never look for a userland (uprobe) handler at all, only the kprobe
> handlers should be considered.
> 
> In this case, the TIF_UPROBE is incorrectly set, and incorrectly (not)
> handled in the "common entry" exit path, which takes us to the infinite
> loop.

This change makes a lot of sense, no reason to check for uprobes if exception
comes from the kernel.

Best regards,
Nam