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Re: Missing unlock_user_struct , Deadlock

From: Laurent Vivier
Subject: Re: Missing unlock_user_struct , Deadlock
Date: Fri, 13 Mar 2020 11:20:22 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.5.0

Le 13/03/2020 à 10:08, Laurent Vivier a écrit :
> Le 13/03/2020 à 05:52, Mansour Ahmadi a écrit :
>> Hi
>> In the case of badframe, user struct is not unlocked here and may lead
>> to deadlock:
>> https://github.com/qemu/qemu/blob/d4f7d56759f7c75270c13d5f3f5f736a9558929c/linux-user/m68k/signal.c#L380
> badframe is called if lock fails, so no need to unlock.
>> In similar case, it in unlocked correctly:
>> https://github.com/qemu/qemu/blob/d4f7d56759f7c75270c13d5f3f5f736a9558929c/linux-user/sh4/signal.c#L303

In fact, this case is a little bit different and wrong but to fix that
the solution is to set frame to NULL when it is declared:


* Unlock an area of guest memory.  The first LEN bytes must be
   flushed back to guest memory. host_ptr = NULL is explicitly
   allowed and does nothing. */
static inline void unlock_user(void *host_ptr, abi_ulong guest_addr,
                               long len)

like for linux-user/aarch64/signal.c:

long do_rt_sigreturn(CPUARMState *env)
    struct target_rt_sigframe *frame = NULL;

We have several targets with the same problem.


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