Re: [PATCH 02/23] tcg: Manage splitwx in tc_ptr_to_region_tree by hand

[Alex Bennée]

Richard Henderson <richard.henderson@linaro.org> writes:

> The use in tcg_tb_lookup is given a random pc that comes from the pc
> of a signal handler.  Do not assert that the pointer is already within
> the code gen buffer at all, much less the writable mirror of it.
>
> Fixes: db0c51a3803
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

OK I bisected to this regression while running:

  "cd builds/bisect && rm -rf * && ../../configure --disable-docs 
--target-list=m68k-linux-user && make -j30 && make check-tcg"

which ultimately fails during the threadcount test for m68k-linux-user.
I'm just testing now to see if that also broke my ARM system test
images.

> ---
>
> For TCI, this indicates a bug in handle_cpu_signal, in that we
> are taking PC from the host signal frame.  Which is, nearly,
> unrelated to TCI at all.
>
> The TCI "pc" is tci_tb_ptr (fixed in the next patch to at least
> be thread-local).  We update this only on calls, since we don't
> expect SEGV during the interpretation loop.  Which works ok for
> softmmu, in which we pass down pc by hand to the helpers, but
> is not ok for user-only, where we simply perform the raw memory
> operation.
>
> I don't know how to fix this, exactly.  Probably by storing to
> tci_tb_ptr before each qemu_ld/qemu_st operation, with barriers.
> Then Doing the Right Thing in handle_cpu_signal.  And perhaps
> by clearing tci_tb_ptr whenever we're not expecting a SEGV on
> behalf of the guest (and thus anything left is a qemu host bug).
>
>
> r~
>
> ---
>  tcg/tcg.c | 23 ++++++++++++++++++++---
>  1 file changed, 20 insertions(+), 3 deletions(-)
>
> diff --git a/tcg/tcg.c b/tcg/tcg.c
> index 9e1b0d73c7..78701cf359 100644
> --- a/tcg/tcg.c
> +++ b/tcg/tcg.c
> @@ -407,11 +407,21 @@ static void tcg_region_trees_init(void)
>      }
>  }
>  
> -static struct tcg_region_tree *tc_ptr_to_region_tree(const void *cp)
> +static struct tcg_region_tree *tc_ptr_to_region_tree(const void *p)
>  {
> -    void *p = tcg_splitwx_to_rw(cp);
>      size_t region_idx;
>  
> +    /*
> +     * Like tcg_splitwx_to_rw, with no assert.  The pc may come from
> +     * a signal handler over which the caller has no control.
> +     */
> +    if (!in_code_gen_buffer(p)) {
> +        p -= tcg_splitwx_diff;
> +        if (!in_code_gen_buffer(p)) {
> +            return NULL;
> +        }
> +    }
> +
>      if (p < region.start_aligned) {
>          region_idx = 0;
>      } else {
> @@ -430,6 +440,7 @@ void tcg_tb_insert(TranslationBlock *tb)
>  {
>      struct tcg_region_tree *rt = tc_ptr_to_region_tree(tb->tc.ptr);
>  
> +    g_assert(rt != NULL);
>      qemu_mutex_lock(&rt->lock);
>      g_tree_insert(rt->tree, &tb->tc, tb);
>      qemu_mutex_unlock(&rt->lock);
> @@ -439,6 +450,7 @@ void tcg_tb_remove(TranslationBlock *tb)
>  {
>      struct tcg_region_tree *rt = tc_ptr_to_region_tree(tb->tc.ptr);
>  
> +    g_assert(rt != NULL);
>      qemu_mutex_lock(&rt->lock);
>      g_tree_remove(rt->tree, &tb->tc);
>      qemu_mutex_unlock(&rt->lock);
> @@ -453,8 +465,13 @@ TranslationBlock *tcg_tb_lookup(uintptr_t tc_ptr)
>  {
>      struct tcg_region_tree *rt = tc_ptr_to_region_tree((void *)tc_ptr);
>      TranslationBlock *tb;
> -    struct tb_tc s = { .ptr = (void *)tc_ptr };
> +    struct tb_tc s;
>  
> +    if (rt == NULL) {
> +        return NULL;
> +    }
> +
> +    s.ptr = (void *)tc_ptr;
>      qemu_mutex_lock(&rt->lock);
>      tb = g_tree_lookup(rt->tree, &s);
>      qemu_mutex_unlock(&rt->lock);


-- 
Alex Bennée