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[3/3] LAM: Support kernel space debugging

Message ID 20240327074739.2969623-4-christina.schimpe@intel.com
State New
Headers
Series Add amd64 LAM watchpoint support |

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Commit Message

Schimpe, Christina March 27, 2024, 7:47 a.m. UTC 
  From: Christina Schimpe <christina.schimpe@intel.com>

Sign-extend watchpoint address for kernel space support.
---
 gdb/amd64-linux-tdep.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

Comments

Willgerodt, Felix April 24, 2024, 11:11 a.m. UTC | #1 
> -----Original Message-----
> From: Schimpe, Christina <christina.schimpe@intel.com>
> Sent: Mittwoch, 27. März 2024 08:48
> To: gdb-patches@sourceware.org
> Cc: Schimpe, Christina <christina.schimpe@intel.com>
> Subject: [PATCH 3/3] LAM: Support kernel space debugging
> 
> From: Christina Schimpe <christina.schimpe@intel.com>
> 
> Sign-extend watchpoint address for kernel space support.
> ---
>  gdb/amd64-linux-tdep.c | 15 ++++++++++++++-
>  1 file changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/gdb/amd64-linux-tdep.c b/gdb/amd64-linux-tdep.c
> index 94c62277623..3498a6d4632 100644
> --- a/gdb/amd64-linux-tdep.c
> +++ b/gdb/amd64-linux-tdep.c
> @@ -51,6 +51,8 @@
> 
>  #include <sstream>
> 
> +/* Bit 63 is used to select between a kernel-space and user-space address.  */
> +#define VA_RANGE_SELECT_BIT_MASK  0x8000000000000000UL
>  #define DEFAULT_TAG_MASK 0xffffffffffffffffULL
> 
>  /* Mapping between the general-purpose registers in `struct user'
> @@ -1852,10 +1854,21 @@ amd64_linux_lam_untag_mask ()
>  static CORE_ADDR
>  amd64_linux_remove_non_addr_bits_wpt (gdbarch *gdbarch, CORE_ADDR
> addr)
>  {
> +  /* The topmost bit tells us if we have a kernel-space address or a user-space
> +     address.  */
> +  const bool kernel_address = (addr & VA_RANGE_SELECT_BIT_MASK) != 0;
> +
> +  CORE_ADDR mask = amd64_linux_lam_untag_mask ();
>    /* Clear insignificant bits of a target address using the untag mask.
>       The untag mask preserves the topmost bit, which distinguishes user space
>       from kernel space address.  */
> -  return (addr & amd64_linux_lam_untag_mask ());
> +  addr &= mask;
> +
> +  /* Sign-extend if we have a kernel-space address.  */
> +  if (kernel_address)
> +    addr |= ~mask;
> +
> +  return addr;

Ah, here is probably where we should add the comment I complained about in
patch 2.

I don't really know much about how kernel space debugging works.
(We can't get a kernel address when doing user-space debugging, can we?)
While this probably doesn't hurt much, I wonder if it isn't just dead code.
Is this file (amd64-linux-tdep.c) even used when doing kernel debugging?
And we still read the mask from /proc/pid/status. Is that available?

Maybe someone with more experience on this can chime in.

Thanks,
Felix
Intel Deutschland GmbH
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Tel: +49 89 99 8853-0, www.intel.de <http://www.intel.de>
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diff mbox series

Patch

diff --git a/gdb/amd64-linux-tdep.c b/gdb/amd64-linux-tdep.c
index 94c62277623..3498a6d4632 100644
--- a/gdb/amd64-linux-tdep.c
+++ b/gdb/amd64-linux-tdep.c
@@ -51,6 +51,8 @@ 
 
 #include <sstream>
 
+/* Bit 63 is used to select between a kernel-space and user-space address.  */
+#define VA_RANGE_SELECT_BIT_MASK  0x8000000000000000UL
 #define DEFAULT_TAG_MASK 0xffffffffffffffffULL
 
 /* Mapping between the general-purpose registers in `struct user'
@@ -1852,10 +1854,21 @@  amd64_linux_lam_untag_mask ()
 static CORE_ADDR
 amd64_linux_remove_non_addr_bits_wpt (gdbarch *gdbarch, CORE_ADDR addr)
 {
+  /* The topmost bit tells us if we have a kernel-space address or a user-space
+     address.  */
+  const bool kernel_address = (addr & VA_RANGE_SELECT_BIT_MASK) != 0;
+
+  CORE_ADDR mask = amd64_linux_lam_untag_mask ();
   /* Clear insignificant bits of a target address using the untag mask.
      The untag mask preserves the topmost bit, which distinguishes user space
      from kernel space address.  */
-  return (addr & amd64_linux_lam_untag_mask ());
+  addr &= mask;
+
+  /* Sign-extend if we have a kernel-space address.  */
+  if (kernel_address)
+    addr |= ~mask;
+
+  return addr;
 }
 
 static void