Re: [PATCH v2] x86/oprofile: remove compat accessors usage from backtrace

[Jan Beulich <jbeulich@xxxxxxxx>]

On 23.04.2021 16:37, Roger Pau Monne wrote:
> Remove the unneeded usage of the compat layer to copy frame pointers
> from guest address space. Instead just use raw_copy_from_guest.
> 
> While there drop the checks for the accessibility of one struct
> frame_head beyond the current one: it's not clear why it's needed and
> all the hypnoses point to dropping such check being harmless. The

DYM "hypotheses"?

> worse that could happen is that a failure happens later if data past
> frame_head is attempted to be fetched, albeit I'm not able to spot any
> such access.
> 
> Also drop the explicit truncation of the head pointer in the 32bit
> case as all callers already pass a zero extended value. The first
> value being rsp from the guest registers,

While I know I'm guilty of splitting hair saying so, I'd like to point
out that I'm unaware of guarantees that the upper halves of GPRs are
zero after a switch from compat to 64-bit mode. With this I'm also
unconvinced there are guarantees that the %rsp stored into a stack
frame is actually guaranteed to be zero-extended. Nevertheless I'm not
meaning this remark to keep the change from going in as is - for all
practical purposes what you say is presumably true.

What I would consider nice though is if the two remaining if() could
be corrected for coding style: Adjacent code is already inconsistent,
so taking the opportunity to move it a little in the right direction
would seem desirable to me. (I would suggest doing so myself while
committing, but because I don't fully agree with dropping the 2-frame
checks described further up without properly understanding why they're
there, I'd like to not put my name on this change in any way, not even
just as committer. But I guess Andrew or Wei or whoever ends up
committing this could do so, as long as they agree of course.)

Jan

> and further calls will use ebp from frame_head_32bit struct.
> 
> Reported-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> ---
> Changes since v2:
>  - Expand commit message.
> ---
>  xen/arch/x86/oprofile/backtrace.c | 26 +++-----------------------
>  1 file changed, 3 insertions(+), 23 deletions(-)
> 
> diff --git a/xen/arch/x86/oprofile/backtrace.c 
> b/xen/arch/x86/oprofile/backtrace.c
> index bd5d1b0f6ce..45f7fb65fa2 100644
> --- a/xen/arch/x86/oprofile/backtrace.c
> +++ b/xen/arch/x86/oprofile/backtrace.c
> @@ -20,7 +20,6 @@ struct __packed frame_head {
>      unsigned long ret;
>  };
>  typedef struct frame_head frame_head_t;
> -DEFINE_XEN_GUEST_HANDLE(frame_head_t);
>  
>  struct __packed frame_head_32bit {
>      uint32_t ebp;
> @@ -43,7 +42,6 @@ dump_hypervisor_backtrace(struct vcpu *vcpu, const struct 
> frame_head *head,
>      return head->ebp;
>  }
>  
> -#ifdef CONFIG_COMPAT
>  static inline int is_32bit_vcpu(struct vcpu *vcpu)
>  {
>      if (is_hvm_vcpu(vcpu))
> @@ -51,7 +49,6 @@ static inline int is_32bit_vcpu(struct vcpu *vcpu)
>      else
>          return is_pv_32bit_vcpu(vcpu);
>  }
> -#endif
>  
>  static struct frame_head *
>  dump_guest_backtrace(struct vcpu *vcpu, const struct frame_head *head,
> @@ -59,34 +56,17 @@ dump_guest_backtrace(struct vcpu *vcpu, const struct 
> frame_head *head,
>  {
>      frame_head_t bufhead;
>  
> -#ifdef CONFIG_COMPAT
>      if ( is_32bit_vcpu(vcpu) )
>      {
> -        DEFINE_COMPAT_HANDLE(frame_head32_t);
> -        __compat_handle_const_frame_head32_t guest_head =
> -            { .c = (unsigned long)head };
>          frame_head32_t bufhead32;
>  
> -        /* Also check accessibility of one struct frame_head beyond */
> -        if (!compat_handle_okay(guest_head, 2))
> -            return 0;
> -        if (__copy_from_compat(&bufhead32, guest_head, 1))
> +        if (raw_copy_from_guest(&bufhead32, head, sizeof(bufhead32)))
>              return 0;
>          bufhead.ebp = (struct frame_head *)(unsigned long)bufhead32.ebp;
>          bufhead.ret = bufhead32.ret;
>      }
> -    else
> -#endif
> -    {
> -        XEN_GUEST_HANDLE_PARAM(const_frame_head_t) guest_head =
> -            const_guest_handle_from_ptr(head, frame_head_t);
> -
> -        /* Also check accessibility of one struct frame_head beyond */
> -        if (!guest_handle_okay(guest_head, 2))
> -            return 0;
> -        if (__copy_from_guest(&bufhead, guest_head, 1))
> -            return 0;
> -    }
> +    else if (raw_copy_from_guest(&bufhead, head, sizeof(bufhead)))
> +        return 0;
>      
>      if (!xenoprof_add_trace(vcpu, bufhead.ret, mode))
>          return 0;
>