Re: [PATCH] [PR gdb/23127] [AArch64] Fix tagged pointer support

Daniel Thompson
2018-05-01T15:02:16Z

On 01/05/18 02:31, Omair Javaid wrote:
> This patch fixes tagged pointer support for AArch64 GDB. Linux kernel debugging
> failure was reported after tagged pointer support was committed.
>
> After a discussion around best path forward to manage tagged pointers on GDB
> side we are going to disable tagged pointers support for aarch64-none-elf-gdb
> because for non-linux applications we cant be sure if tagged pointers will be
> used by MMU or not.
>
> Also for aarch64-linux-gdb we are going to sign extend user-space address after
> clearing tag bits. This will help us debug both kernel and user-space addresses
> based on information from linux kernel documentation given below:
>
> According to AArch64 memory map:
> https://www.kernel.org/doc/Documentation/arm64/memory.txt
>
> "User addresses have bits 63:48 set to 0 while the kernel addresses have
> the same bits set to 1."
>
> According to AArch64 tagged pointers document:
> https://www.kernel.org/doc/Documentation/arm64/tagged-pointers.txt
>
> The kernel configures the translation tables so that translations made
> via TTBR0 (i.e. userspace mappings) have the top byte (bits 63:56) of
> the virtual address ignored by the translation hardware. This frees up
> this byte for application use.
>
> Running gdb testsuite after applying this patch introduces no regressions and
> tagged pointer test cases still pass.

... and I kicked the tyres a little bit using kgdb.

print worked as expected, backtrace no longer provokes a gdb panic and
breakpoints work (albeit for rather approximate definition of work...
and the need for approximation is not gdb's fault).

Daniel.

> gdb/ChangeLog:
> 2018-05-01 Omair Javaid <omair.javaid@linaro.org>
>
> * aarch64-linux-tdep.c (aarch64_linux_init_abi): Add call to
> set_gdbarch_significant_addr_bit.
> * aarch64-tdep.c (aarch64_gdbarch_init): Remove call to
> set_gdbarch_significant_addr_bit.
> * utils.c (address_significant): Update to sign extend addr.
> ---
> gdb/aarch64-linux-tdep.c | 5 +++++
> gdb/aarch64-tdep.c | 5 -----
> gdb/utils.c | 14 +++++++++-----
> 3 files changed, 14 insertions(+), 10 deletions(-)
>
> diff --git a/gdb/aarch64-linux-tdep.c b/gdb/aarch64-linux-tdep.c
> index 1f3e888..ba5757d 100644
> --- a/gdb/aarch64-linux-tdep.c
> +++ b/gdb/aarch64-linux-tdep.c
> @@ -1062,6 +1062,11 @@ aarch64_linux_init_abi (struct gdbarch_info info, struct gdbarch *gdbarch)
> /* Syscall record. */
> tdep->aarch64_syscall_record = aarch64_linux_syscall_record;
>
> + /* The top byte of a user space address known as the "tag",
> + is ignored by the kernel and can be regarded as additional
> + data associated with the address. */
> + set_gdbarch_significant_addr_bit (gdbarch, 56);
> +
> /* Initialize the aarch64_linux_record_tdep. */
> /* These values are the size of the type that will be used in a system
> call. They are obtained from Linux Kernel source. */
> diff --git a/gdb/aarch64-tdep.c b/gdb/aarch64-tdep.c
> index 01566b4..3c1f389 100644
> --- a/gdb/aarch64-tdep.c
> +++ b/gdb/aarch64-tdep.c
> @@ -2972,11 +2972,6 @@ aarch64_gdbarch_init (struct gdbarch_info info, struct gdbarch_list *arches)
> set_tdesc_pseudo_register_reggroup_p (gdbarch,
> aarch64_pseudo_register_reggroup_p);
>
> - /* The top byte of an address is known as the "tag" and is
> - ignored by the kernel, the hardware, etc. and can be regarded
> - as additional data associated with the address. */
> - set_gdbarch_significant_addr_bit (gdbarch, 56);
> -
> /* ABI */
> set_gdbarch_short_bit (gdbarch, 16);
> set_gdbarch_int_bit (gdbarch, 32);
> diff --git a/gdb/utils.c b/gdb/utils.c
> index b957b0d..1f9be8f 100644
> --- a/gdb/utils.c
> +++ b/gdb/utils.c
> @@ -2704,14 +2704,18 @@ When set, debugging messages will be marked with seconds and microseconds."),
> CORE_ADDR
> address_significant (gdbarch *gdbarch, CORE_ADDR addr)
> {
> - /* Truncate address to the significant bits of a target address,
> - avoiding shifts larger or equal than the width of a CORE_ADDR.
> - The local variable ADDR_BIT stops the compiler reporting a shift
> - overflow when it won't occur. */
> + /* Clear insignificant bits of a target address and sign extend resulting
> + address, avoiding shifts larger or equal than the width of a CORE_ADDR.
> + The local variable ADDR_BIT stops the compiler reporting a shift overflow
> + when it won't occur. */
> int addr_bit = gdbarch_significant_addr_bit (gdbarch);
>
> if (addr_bit < (sizeof (CORE_ADDR) * HOST_CHAR_BIT))
> - addr &= ((CORE_ADDR) 1 << addr_bit) - 1;
> + {
> + CORE_ADDR sign = (CORE_ADDR) 1 << (addr_bit - 1);
> + addr &= ((CORE_ADDR) 1 << addr_bit) - 1;
> + addr = (addr ^ sign) - sign;
> + }
>
> return addr;
> }
>