x86_64-windows GDB crash due to fs_base/gs_base registers

GDB is currently crashing anytime we try to access the fs_base/gs_base
registers, either to read them, or to write them. This can be observed
under various scenarios:
  - Explicit reference to those registers (eg: print $fs_base) --
    probably relatively rare;
  - Calling a function in the inferior, with the crash happening
    because we are trying to read those registers in order to save
    their value ahead of making the function call;
  - Just a plain "info registers";

The crash was introduced by the following commit:

    | commit 48aeef91c248291dd03583798904612426b1f40a
    | Date:   Mon Jun 26 18:14:43 2017 -0700
    | Subject: Include the fs_base and gs_base registers in amd64 target descriptions.

The Windows-nat implementation was unfortunately not prepared to deal
with those new registers. In particular, the way it fetches registers
is done by using a table where the index is the register number, and
the value at that index is the offset in the area in the thread's CONTEXT
data where the corresponding register value is stored.

For instance, in amd64-windows-nat.c, we can find the mappings static
array containing the following 57 elements in it:

    #define context_offset(x) (offsetof (CONTEXT, x))
    static const int mappings[] =
    {
      context_offset (Rax),
      [...]
      context_offset (FloatSave.MxCsr)
    };

That array is then used by windows_fetch_one_register via:

    char *context_offset = ((char *) &th->context) + mappings[r];

The problem is that fs_base's register number is 172, which is
well past the end of the mappings array (57 elements in total).
We end up getting an undefined offset, which happens to be so large
that it then causes the address where we try to read the register
value (a little bit later) to be invalid, thus crashing GDB with
a SEGV.

This patch side-steps the issue entirely by removing support for
those registers in GDB on x86_64-windows, because a look at the
CONTEXT structure indicates no support for getting those registers.

A more comprehensive fix would patch the potential buffer overflow
of the mappings array, but this can be done as a separate commit.

gdb/ChangeLog:

        * gdb/amd64-tdep.h (amd64_create_target_description): Add
        "segments" parameter.
        * gdb/amd64-tdep.c (amd64_none_init_abi, amd64_x32_none_init_abi)
        (_initialize_amd64_tdep): Update call to
        amd64_create_target_description.
        (amd64_target_description): Add "segments" parameter.  Adjust
        the implementation to use it.
        * gdb/amd64-linux-tdep.c (amd64_linux_read_description): Update
        call to amd64_create_target_description.
        * gdb/amd64-windows-tdep.c (amd64_windows_init_abi): Likewise.
        * gdb/arch/amd64.h (amd64_create_target_description): Add
        "segments" register.
        * gdb/arch/amd64.c (amd64_create_target_description): Add
        "segments" parameter.  Call create_feature_i386_64bit_segments
        only if SEGMENTS is true.
        * gdb/gdbserver/win32-i386-low.c (i386_arch_setup): Update
        call to amd64_create_target_description.

Tested on x86_64-windows using AdaCore's testsuite (by Joel Brobecker
<brobecker at adacore dot com>).

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 0cbccf5f8a66960bcacb954fe31a688e0bc4bda6..c47c111466d393fb20fa766297e4e489175dc952 100644 (file)
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,23 @@
+2018-06-29  Pedro Alves  <palves@redhat.com>
+
+       * gdb/amd64-tdep.h (amd64_create_target_description): Add
+       "segments" parameter.
+       * gdb/amd64-tdep.c (amd64_none_init_abi, amd64_x32_none_init_abi)
+       (_initialize_amd64_tdep): Update call to
+       amd64_create_target_description.
+       (amd64_target_description): Add "segments" parameter.  Adjust
+       the implementation to use it.
+       * gdb/amd64-linux-tdep.c (amd64_linux_read_description): Update
+       call to amd64_create_target_description.
+       * gdb/amd64-windows-tdep.c (amd64_windows_init_abi): Likewise.
+       * gdb/arch/amd64.h (amd64_create_target_description): Add
+       "segments" register.
+       * gdb/arch/amd64.c (amd64_create_target_description): Add
+       "segments" parameter.  Call create_feature_i386_64bit_segments
+       only if SEGMENTS is true.
+       * gdb/gdbserver/win32-i386-low.c (i386_arch_setup): Update
+       call to amd64_create_target_description.
+
 2018-06-29  Pedro Alves  <palves@redhat.com>
 
        * thread.c (thread_target_id_str): New, factored out from ...

diff --git a/gdb/amd64-linux-tdep.c b/gdb/amd64-linux-tdep.c
index ef9248d708d35724a97500aacf04e0a07830fff3..7fe37d83f696bf2f3ebeff372488729879a7ae52 100644 (file)
--- a/gdb/amd64-linux-tdep.c
+++ b/gdb/amd64-linux-tdep.c
@@ -1594,7 +1594,8 @@ amd64_linux_read_description (uint64_t xcr0_features_bit, bool is_x32)
     }
 
   if (*tdesc == NULL)
-    *tdesc = amd64_create_target_description (xcr0_features_bit, is_x32, true);
+    *tdesc = amd64_create_target_description (xcr0_features_bit, is_x32,
+                                             true, true);
 
   return *tdesc;
 }

diff --git a/gdb/amd64-tdep.c b/gdb/amd64-tdep.c
index 9f8f018dd155f3f6db3a838e8d582149e5d7854c..190e086ebfe3abcfd4346db4214d7b8d65607571 100644 (file)
--- a/gdb/amd64-tdep.c
+++ b/gdb/amd64-tdep.c
@@ -3225,7 +3225,8 @@ amd64_init_abi (struct gdbarch_info info, struct gdbarch *gdbarch,
 static void
 amd64_none_init_abi (gdbarch_info info, gdbarch *arch)
 {
-  amd64_init_abi (info, arch, amd64_target_description (X86_XSTATE_SSE_MASK));
+  amd64_init_abi (info, arch, amd64_target_description (X86_XSTATE_SSE_MASK,
+                                                       true));
 }
 
 static struct type *
@@ -3266,25 +3267,27 @@ static void
 amd64_x32_none_init_abi (gdbarch_info info, gdbarch *arch)
 {
   amd64_x32_init_abi (info, arch,
-                     amd64_target_description (X86_XSTATE_SSE_MASK));
+                     amd64_target_description (X86_XSTATE_SSE_MASK, true));
 }
 
 /* Return the target description for a specified XSAVE feature mask.  */
 
 const struct target_desc *
-amd64_target_description (uint64_t xcr0)
+amd64_target_description (uint64_t xcr0, bool segments)
 {
   static target_desc *amd64_tdescs \
-    [2/*AVX*/][2/*MPX*/][2/*AVX512*/][2/*PKRU*/] = {};
+    [2/*AVX*/][2/*MPX*/][2/*AVX512*/][2/*PKRU*/][2/*segments*/] = {};
   target_desc **tdesc;
 
   tdesc = &amd64_tdescs[(xcr0 & X86_XSTATE_AVX) ? 1 : 0]
     [(xcr0 & X86_XSTATE_MPX) ? 1 : 0]
     [(xcr0 & X86_XSTATE_AVX512) ? 1 : 0]
-    [(xcr0 & X86_XSTATE_PKRU) ? 1 : 0];
+    [(xcr0 & X86_XSTATE_PKRU) ? 1 : 0]
+    [segments ? 1 : 0];
 
   if (*tdesc == NULL)
-    *tdesc = amd64_create_target_description (xcr0, false, false);
+    *tdesc = amd64_create_target_description (xcr0, false, false,
+                                             segments);
 
   return *tdesc;
 }
@@ -3314,7 +3317,7 @@ _initialize_amd64_tdep (void)
 
   for (auto &a : xml_masks)
     {
-      auto tdesc = amd64_target_description (a.mask);
+      auto tdesc = amd64_target_description (a.mask, true);
 
       selftests::record_xml_tdesc (a.xml, tdesc);
     }

diff --git a/gdb/amd64-tdep.h b/gdb/amd64-tdep.h
index 7d3791ad9368908499ac4ba6c8ec6ca71b222c1a..94e012632ff55cd7b4d2c2c10969e4c76e4627b9 100644 (file)
--- a/gdb/amd64-tdep.h
+++ b/gdb/amd64-tdep.h
@@ -106,7 +106,8 @@ extern void amd64_init_abi (struct gdbarch_info info,
 extern void amd64_x32_init_abi (struct gdbarch_info info,
                                struct gdbarch *gdbarch,
                                const target_desc *default_tdesc);
-extern const struct target_desc *amd64_target_description (uint64_t xcr0);
+extern const struct target_desc *amd64_target_description (uint64_t xcr0,
+                                                          bool segments);
 
 /* Fill register REGNUM in REGCACHE with the appropriate
    floating-point or SSE register value from *FXSAVE.  If REGNUM is

diff --git a/gdb/amd64-windows-tdep.c b/gdb/amd64-windows-tdep.c
index 83a7f2f32ed2a28650dda0df6697e90eb391b8cd..904875baccd825eedac9d9571041671f163acc0e 100644 (file)
--- a/gdb/amd64-windows-tdep.c
+++ b/gdb/amd64-windows-tdep.c
@@ -1226,7 +1226,7 @@ amd64_windows_init_abi (struct gdbarch_info info, struct gdbarch *gdbarch)
   frame_unwind_append_unwinder (gdbarch, &amd64_windows_frame_unwind);
 
   amd64_init_abi (info, gdbarch,
-                 amd64_target_description (X86_XSTATE_SSE_MASK));
+                 amd64_target_description (X86_XSTATE_SSE_MASK, false));
 
   windows_init_abi (info, gdbarch);
 

diff --git a/gdb/arch/amd64.c b/gdb/arch/amd64.c
index d31d8f1f75a77946127839a72bdc950d2bcbfefb..4ad17c3fdcd599313a654a4426101c0046a5a133 100644 (file)
--- a/gdb/arch/amd64.c
+++ b/gdb/arch/amd64.c
@@ -33,10 +33,12 @@
 
 /* Create amd64 target descriptions according to XCR0.  If IS_X32 is
    true, create the x32 ones.  If IS_LINUX is true, create target
-   descriptions for Linux.  */
+   descriptions for Linux.  If SEGMENTS is true, then include
+   the "org.gnu.gdb.i386.segments" feature registers.  */
 
 target_desc *
-amd64_create_target_description (uint64_t xcr0, bool is_x32, bool is_linux)
+amd64_create_target_description (uint64_t xcr0, bool is_x32, bool is_linux,
+                                bool segments)
 {
   target_desc *tdesc = allocate_target_description ();
 
@@ -57,7 +59,8 @@ amd64_create_target_description (uint64_t xcr0, bool is_x32, bool is_linux)
   regnum = create_feature_i386_64bit_sse (tdesc, regnum);
   if (is_linux)
     regnum = create_feature_i386_64bit_linux (tdesc, regnum);
-  regnum = create_feature_i386_64bit_segments (tdesc, regnum);
+  if (segments)
+    regnum = create_feature_i386_64bit_segments (tdesc, regnum);
 
   if (xcr0 & X86_XSTATE_AVX)
     regnum = create_feature_i386_64bit_avx (tdesc, regnum);

diff --git a/gdb/arch/amd64.h b/gdb/arch/amd64.h
index c0c4dc27efe62ca834d00e9f77066bd6338a49bd..4a659657da1972206813650eb9d370a2f2e06ed3 100644 (file)
--- a/gdb/arch/amd64.h
+++ b/gdb/arch/amd64.h
@@ -19,4 +19,4 @@
 #include <stdint.h>
 
 target_desc *amd64_create_target_description (uint64_t xcr0, bool is_x32,
-                                             bool is_linux);
+                                             bool is_linux, bool segments);

diff --git a/gdb/gdbserver/win32-i386-low.c b/gdb/gdbserver/win32-i386-low.c
index 16fe2c85b2a081d45112cf2225f653cf3dbc7fba..62221688cbdbfd825aa3a1727ae182686a88202c 100644 (file)
--- a/gdb/gdbserver/win32-i386-low.c
+++ b/gdb/gdbserver/win32-i386-low.c
@@ -436,7 +436,7 @@ i386_arch_setup (void)
 
 #ifdef __x86_64__
   tdesc = amd64_create_target_description (X86_XSTATE_SSE_MASK, false,
-                                                false);
+                                          false, false);
   const char **expedite_regs = amd64_expedite_regs;
 #else
   tdesc = i386_create_target_description (X86_XSTATE_SSE_MASK, false);