Re: [PATCH v2 3/4] semihosting/arm-compat-semi: deref parameter register for SYS_HEAPINFO

[Peter Maydell]

On Tue, 9 Mar 2021 at 14:23, Alex Bennée <alex.bennee@linaro.org> wrote:
>
> As per the spec:
>
>   the PARAMETER REGISTER contains the address of a pointer to a
>   four-field data block.
>
> So we need to follow the pointer and place the results of SYS_HEAPINFO
> there.
>
> Bug: https://bugs.launchpad.net/bugs/1915925
> Cc: Bug 1915925 <1915925@bugs.launchpad.net>
> Cc: Keith Packard <keithp@keithp.com>
> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
> ---
>  semihosting/arm-compat-semi.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/semihosting/arm-compat-semi.c b/semihosting/arm-compat-semi.c
> index 733eea1e2d..2ac9226d29 100644
> --- a/semihosting/arm-compat-semi.c
> +++ b/semihosting/arm-compat-semi.c
> @@ -1210,6 +1210,8 @@ target_ulong do_common_semihosting(CPUState *cs)
>              retvals[2] = rambase + limit; /* Stack base */
>              retvals[3] = rambase; /* Stack limit.  */
>  #endif
> +            /* The result array is pointed to by arg0 */
> +            args = arg0;
>
>              for (i = 0; i < ARRAY_SIZE(retvals); i++) {
>                  bool fail;
> --

No, 'args' is the argument array. That's not the same thing
as the data block we're writing, and we shouldn't reassign
that variable here.

What was wrong with the old arm-semi.c code, which just did
appropriate loads and stores here, and worked fine and was
not buggy ?

thanks
-- PMM