Re: [PATCH v2 05/74] semihosting: Add target_strlen for softmmu-uaccess.h

[Peter Maydell]

On Tue, 3 May 2022 at 20:57, Richard Henderson
<richard.henderson@linaro.org> wrote:
>
> Mirror the user-only function of the same name.
>
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> ---
>  include/semihosting/softmmu-uaccess.h |  3 +++
>  semihosting/uaccess.c                 | 29 +++++++++++++++++++++++++++
>  2 files changed, 32 insertions(+)
>
> diff --git a/include/semihosting/softmmu-uaccess.h 
> b/include/semihosting/softmmu-uaccess.h
> index 03300376d3..4f08dfc098 100644
> --- a/include/semihosting/softmmu-uaccess.h
> +++ b/include/semihosting/softmmu-uaccess.h
> @@ -53,4 +53,7 @@ void softmmu_unlock_user(CPUArchState *env, void *p,
>                           target_ulong addr, target_ulong len);
>  #define unlock_user(s, args, len) softmmu_unlock_user(env, s, args, len)
>
> +ssize_t softmmu_strlen_user(CPUArchState *env, target_ulong addr);
> +#define target_strlen(p) softmmu_strlen_user(env, p)
> +
>  #endif /* SEMIHOSTING_SOFTMMU_UACCESS_H */
> diff --git a/semihosting/uaccess.c b/semihosting/uaccess.c
> index 0d3b32b75d..3cd809122c 100644
> --- a/semihosting/uaccess.c
> +++ b/semihosting/uaccess.c
> @@ -23,6 +23,35 @@ void *softmmu_lock_user(CPUArchState *env, target_ulong 
> addr,
>      return p;
>  }
>
> +ssize_t softmmu_strlen_user(CPUArchState *env, target_ulong addr)
> +{
> +    char buf[256];
> +    size_t len = 0;
> +
> +    while (1) {
> +        size_t chunk;
> +        char *p;
> +
> +        chunk = -(addr | TARGET_PAGE_MASK);

'chunk' is unsigned but we're assigning it a negative number here...
I assume this is doing some clever bit-twiddling trick but it isn't
very obvious.

In any case, rounding to a page boundary isn't sufficient to
avoid problems, because we don't mandate that RAM-to-device
MemoryRegion boundaries happen on page boundaries. I think you
either need to do this at a low enough level that you can
look at what MemoryRegion types you're reading from, or you need
to do it byte at a time.

> +        chunk = MIN(chunk, sizeof(buf));
> +
> +        if (cpu_memory_rw_debug(env_cpu(env), addr, buf, chunk, 0)) {
> +            return -1;
> +        }
> +        p = memchr(buf, 0, chunk);
> +        if (p) {
> +            len += p - buf;
> +            return len <= INT32_MAX ? (ssize_t)len : -1;
> +        }
> +
> +        len += chunk;
> +        addr += chunk;
> +        if (len > INT32_MAX) {
> +            return -1;
> +        }
> +    }
> +}

thanks
-- PMM