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

["Alex Bennée" <alex.bennee@linaro.org>]

Alex Bennée <alex.bennee@linaro.org> writes:

> 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 my ARM test system:

./qemu-system-aarch64 -machine virt,virtualization=on -cpu cortex-a57
-serial mon:stdio -nic user,model=virtio-net-pci,hostfwd=tcp::2222-:22
-device virtio-scsi-pci -drive
file=/dev/zvol/hackpool-0/debian-bullseye-arm64,id=hd0,index=0,if=none,format=raw
-device scsi-hd,drive=hd0 -display none -m 4096 -smp 4 -drive
if=pflash,file=/usr/share/AAVMF/AAVMF_CODE.fd,format=raw,readonly -drive
if=pflash,file=$HOME/images/AAVMF_VARS.fd,format=raw


Yep with this:

  [    2.948980] Checked W+X mappings: passed, no W+X pages found
  [    2.949443] Run /init as init process
  [    2.959082] Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b
  [    2.959563] CPU: 3 PID: 1 Comm: init Not tainted 5.10.0-1-arm64 #1 Debian 5.10.4-1
  [    2.959768] Hardware name: QEMU QEMU Virtual Machine, BIOS 0.0.0 02/06/2015
  [    2.960144] Call trace:
  [    2.960267]  dump_backtrace+0x0/0x1e4
  [    2.960491]  show_stack+0x24/0x6c
  [    2.960699]  dump_stack+0xd0/0x12c
  [    2.960862]  panic+0x168/0x370
  [    2.960990]  do_exit+0x9a8/0x9c0
  [    2.961072]  do_group_exit+0x44/0xac
  [    2.961163]  get_signal+0x174/0x910
  [    2.961256]  do_notify_resume+0x22c/0x9a0
  [    2.961355]  work_pending+0xc/0x39c
  [    2.961849] SMP: stopping secondary CPUs
  [    2.962341] Kernel Offset: disabled
  [    2.962585] CPU features: 0x0240022,61006082
  [    2.962729] Memory Limit: none
  [    2.963158] ---[ end Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b ]---
  QEMU 5.2.50 monitor - type 'help' for more information
  (qemu) quit

Where as I can boot from the commit before....

>
>> ---
>>
>> 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