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Re: [PATCH V3 19/23] xen/arm: io: Abstract sign-extension




On 30.11.20 23:03, Volodymyr Babchuk wrote:
Hi,

Hi Volodymyr



Oleksandr Tyshchenko writes:

From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>

In order to avoid code duplication (both handle_read() and
handle_ioserv() contain the same code for the sign-extension)
put this code to a common helper to be used for both.

Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
CC: Julien Grall <julien.grall@xxxxxxx>

---
Please note, this is a split/cleanup/hardening of Julien's PoC:
"Add support for Guest IO forwarding to a device emulator"

Changes V1 -> V2:
    - new patch

Changes V2 -> V3:
    - no changes
---
---
  xen/arch/arm/io.c           | 18 ++----------------
  xen/arch/arm/ioreq.c        | 17 +----------------
  xen/include/asm-arm/traps.h | 24 ++++++++++++++++++++++++
  3 files changed, 27 insertions(+), 32 deletions(-)

diff --git a/xen/arch/arm/io.c b/xen/arch/arm/io.c
index f44cfd4..8d6ec6c 100644
--- a/xen/arch/arm/io.c
+++ b/xen/arch/arm/io.c
@@ -23,6 +23,7 @@
  #include <asm/cpuerrata.h>
  #include <asm/current.h>
  #include <asm/mmio.h>
+#include <asm/traps.h>
  #include <asm/hvm/ioreq.h>
#include "decode.h"
@@ -39,26 +40,11 @@ static enum io_state handle_read(const struct mmio_handler 
*handler,
       * setting r).
       */
      register_t r = 0;
-    uint8_t size = (1 << dabt.size) * 8;
if ( !handler->ops->read(v, info, &r, handler->priv) )
          return IO_ABORT;
- /*
-     * Sign extend if required.
-     * Note that we expect the read handler to have zeroed the bits
-     * outside the requested access size.
-     */
-    if ( dabt.sign && (r & (1UL << (size - 1))) )
-    {
-        /*
-         * We are relying on register_t using the same as
-         * an unsigned long in order to keep the 32-bit assembly
-         * code smaller.
-         */
-        BUILD_BUG_ON(sizeof(register_t) != sizeof(unsigned long));
-        r |= (~0UL) << size;
-    }
+    r = sign_extend(dabt, r);
set_user_reg(regs, dabt.reg, r); diff --git a/xen/arch/arm/ioreq.c b/xen/arch/arm/ioreq.c
index f08190c..2f39289 100644
--- a/xen/arch/arm/ioreq.c
+++ b/xen/arch/arm/ioreq.c
@@ -28,7 +28,6 @@ enum io_state handle_ioserv(struct cpu_user_regs *regs, 
struct vcpu *v)
      const union hsr hsr = { .bits = regs->hsr };
      const struct hsr_dabt dabt = hsr.dabt;
      /* Code is similar to handle_read */
-    uint8_t size = (1 << dabt.size) * 8;
      register_t r = v->io.req.data;
/* We are done with the IO */
@@ -37,21 +36,7 @@ enum io_state handle_ioserv(struct cpu_user_regs *regs, 
struct vcpu *v)
      if ( dabt.write )
          return IO_HANDLED;
- /*
-     * Sign extend if required.
-     * Note that we expect the read handler to have zeroed the bits
-     * outside the requested access size.
-     */
-    if ( dabt.sign && (r & (1UL << (size - 1))) )
-    {
-        /*
-         * We are relying on register_t using the same as
-         * an unsigned long in order to keep the 32-bit assembly
-         * code smaller.
-         */
-        BUILD_BUG_ON(sizeof(register_t) != sizeof(unsigned long));
-        r |= (~0UL) << size;
-    }
+    r = sign_extend(dabt, r);
set_user_reg(regs, dabt.reg, r); diff --git a/xen/include/asm-arm/traps.h b/xen/include/asm-arm/traps.h
index 997c378..e301c44 100644
--- a/xen/include/asm-arm/traps.h
+++ b/xen/include/asm-arm/traps.h
@@ -83,6 +83,30 @@ static inline bool VABORT_GEN_BY_GUEST(const struct 
cpu_user_regs *regs)
          (unsigned long)abort_guest_exit_end == regs->pc;
  }
+/* Check whether the sign extension is required and perform it */
+static inline register_t sign_extend(const struct hsr_dabt dabt, register_t r)
+{
+    uint8_t size = (1 << dabt.size) * 8;
+
+    /*
+     * Sign extend if required.
+     * Note that we expect the read handler to have zeroed the bits
+     * outside the requested access size.
+     */
+    if ( dabt.sign && (r & (1UL << (size - 1))) )
+    {
+        /*
+         * We are relying on register_t using the same as
+         * an unsigned long in order to keep the 32-bit assembly
+         * code smaller.
+         */
+        BUILD_BUG_ON(sizeof(register_t) != sizeof(unsigned long));
+        r |= (~0UL) << size;
If `size` is 64, you will get undefined behavior there.
I think, we don't need to worry about undefined behavior here. Having size=64 would be possible with doubleword (dabt.size=3). But if "r" adjustment gets called (I mean Syndrome Sign Extend bit is set) then we deal with byte, halfword or word operations (dabt.size<3). Or I missed something?


--
Regards,

Oleksandr Tyshchenko




 


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