[PATCH RFC] linux-user: Allow gdbstub to ignore page protection

[Ilya Leoshkevich]

Hi,

I've noticed that gdbstub behaves differently from gdbserver in that it
doesn't allow reading non-readable pages. The below patch works for me,
but I'm not sure if I haven't introduced layering violations or race
conditions here, so I'm sending this as an RFC. Please let me know if
the code can be improved.

Best regards,
Ilya



gdbserver ignores page protection (by virtue of using /proc/$pid/mem),
so qemu gdbstub should too. One option would be to use the same
mechanism, but this will not work if /proc is not mounted. So
temporarily set the necessary protection bits instead.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
---
 cpu-target.c | 34 ++++++++++++++++++++++++++++------
 1 file changed, 28 insertions(+), 6 deletions(-)

diff --git a/cpu-target.c b/cpu-target.c
index 508013e23d2..8dd7342705b 100644
--- a/cpu-target.c
+++ b/cpu-target.c
@@ -27,6 +27,7 @@
 #include "migration/vmstate.h"
 #ifdef CONFIG_USER_ONLY
 #include "qemu.h"
+#include "user-mmap.h"
 #else
 #include "hw/core/sysemu-cpu-ops.h"
 #include "exec/address-spaces.h"
@@ -368,6 +369,7 @@ int cpu_memory_rw_debug(CPUState *cpu, vaddr addr,
     vaddr l, page;
     void * p;
     uint8_t *buf = ptr;
+    int prot;
 
     while (len > 0) {
         page = addr & TARGET_PAGE_MASK;
@@ -377,22 +379,42 @@ int cpu_memory_rw_debug(CPUState *cpu, vaddr addr,
         flags = page_get_flags(page);
         if (!(flags & PAGE_VALID))
             return -1;
+        prot = ((flags & PAGE_READ) ? PROT_READ : 0) |
+               ((flags & PAGE_WRITE) ? PROT_WRITE : 0) |
+               ((flags & PAGE_EXEC) ? PROT_EXEC : 0);
         if (is_write) {
-            if (!(flags & PAGE_WRITE))
-                return -1;
+            if (!(prot & PROT_WRITE)) {
+                if (target_mprotect(page, TARGET_PAGE_SIZE,
+                                    prot | PROT_WRITE)) {
+                    return -1;
+                }
+            }
             /* XXX: this code should not depend on lock_user */
-            if (!(p = lock_user(VERIFY_WRITE, addr, l, 0)))
+            if (!(p = lock_user(VERIFY_NONE, addr, l, 0)))
                 return -1;
             memcpy(p, buf, l);
             unlock_user(p, addr, l);
+            if (!(prot & PROT_WRITE)) {
+                if (target_mprotect(page, TARGET_PAGE_SIZE, prot)) {
+                    return -1;
+                }
+            }
         } else {
-            if (!(flags & PAGE_READ))
-                return -1;
+            if (!(prot & PROT_READ)) {
+                if (target_mprotect(page, TARGET_PAGE_SIZE, prot | PROT_READ)) 
{
+                    return -1;
+                }
+            }
             /* XXX: this code should not depend on lock_user */
-            if (!(p = lock_user(VERIFY_READ, addr, l, 1)))
+            if (!(p = lock_user(VERIFY_NONE, addr, l, 1)))
                 return -1;
             memcpy(buf, p, l);
             unlock_user(p, addr, 0);
+            if (!(prot & PROT_READ)) {
+                if (target_mprotect(page, TARGET_PAGE_SIZE, prot)) {
+                    return -1;
+                }
+            }
         }
         len -= l;
         buf += l;
-- 
2.43.0