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binutils-gdb/gdb/x86-linux-nat.c
Tom de Vries fbf3c4b979 [gdb/tdep] Use pid to choose process 64/32-bitness 
In a linux kernel mailing list discussion, it was mentioned that "gdb has
this odd thing where it takes the 64-bit vs 32-bit data for the whole process
from one thread, and picks the worst possible thread to do it (ie explicitly
not even the main thread, ...)" [1].

The picking of the thread is done here in
x86_linux_nat_target::read_description:
...
  /* GNU/Linux LWP ID's are process ID's.  */
  tid = inferior_ptid.lwp ();
  if (tid == 0)
    tid = inferior_ptid.pid (); /* Not a threaded program.  */
...

To understand what this code does, let's investigate a scenario in which
inferior_ptid.lwp () != inferior_ptid.pid ().

Say we start exec jit-attach-pie, identified with pid x.  The main thread
starts another thread that sleeps, and then the main thread waits for the
sleeping thread.  So we have two threads, identified with LWP IDs x and x+1:
...
PID  LWP  CMD
x    x    ./jit-attach-pie
x    x+1  ./jit-attach-pie
...
[ The thread with LWP x is known as the thread group leader. ]

When attaching to this exec using the pid, gdb does a stop_all_threads which
iterates over all the threads, first LWP x, and then LWP x+1.

So the state we arrive with at x86_linux_nat_target::read_description is:
...
(gdb) p inferior_ptid
$1 = {m_pid = x, m_lwp = x+1, m_tid = 0}
...
and consequently we probe 64/32-bitness from thread LWP x+1.

[ Note that this is different from when gdb doesn't attach but instead
launches the exec itself, in which case there's just one thread to begin with,
and consequently the probed thread is LWP x. ]

According to aforementioned remark, a better choice would have been the main
thread, that is, LWP x.

This patch implement that choice, by simply doing:
...
  tid = inferior_ptid.pid ();
...

The fact that gdb makes a per-process permanent choice for 64/32-bitness is a
problem in itself: each thread can be in either 64 or 32 bit mode, and change
forth and back.  That is a problem that this patch doesn't fix.

Now finally: why does this matter in the context of the linux kernel
discussion?  The discussion was related to a patch that exposed io_uring
threads to user-space.  This made it possible that one of those threads would
be picked out to select 64/32-bitness.  Given that such threads are atypical
user-space threads in the sense that they don't return to user-space and don't
have a userspace register state, reading their registers returns garbage, and
so it could f.i. occur that in a 64-bit process with all normal user-space
threads in 64-bit mode, the probing would return 32-bit.

It may be that this is worked-around on the kernel side by providing userspace
register state in those threads such that current gdb is happy.  Nevertheless,
it seems prudent to fix this on the gdb size as well.

Tested on x86_64-linux.

[1] https://lore.kernel.org/io-uring/CAHk-=wh0KoEZXPYMGkfkeVEerSCEF1AiCZSvz9TRrx=Kj74D+Q@mail.gmail.com/

gdb/ChangeLog:

2021-05-23  Tom de Vries  <tdevries@suse.de>

	PR tdep/27822
	* target.h (struct target_ops): Mention target_thread_architecture in
	read_description comment.
	* x86-linux-nat.c (x86_linux_nat_target::read_description): Use
	pid to determine if process is 64-bit or 32-bit.
	* aarch64-linux-nat.c (aarch64_linux_nat_target::read_description):
	Same.
	* ppc-linux-nat.c (ppc_linux_nat_target::read_description): Same.
        * riscv-linux-nat.c (riscv_linux_nat_target::read_description): Same.
	* s390-linux-nat.c (s390_linux_nat_target::read_description): Same.
	* arm-linux-nat.c (arm_linux_nat_target::read_description): Same.
	Likewise, use pid to determine if kernel supports reading VFP
	registers.
2021-05-23 10:08:45 +02:00

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/* Native-dependent code for GNU/Linux x86 (i386 and x86-64).
Copyright (C) 1999-2021 Free Software Foundation, Inc.
This file is part of GDB.
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 3 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program. If not, see <http://www.gnu.org/licenses/>. */
#include "defs.h"
#include "inferior.h"
#include "elf/common.h"
#include "gdb_proc_service.h"
#include "nat/gdb_ptrace.h"
#include <sys/user.h>
#include <sys/procfs.h>
#include <sys/uio.h>
#include "x86-nat.h"
#ifndef __x86_64__
#include "i386-linux-nat.h"
#endif
#include "x86-linux-nat.h"
#include "i386-linux-tdep.h"
#ifdef __x86_64__
#include "amd64-linux-tdep.h"
#endif
#include "gdbsupport/x86-xstate.h"
#include "nat/linux-btrace.h"
#include "nat/linux-nat.h"
#include "nat/x86-linux.h"
#include "nat/x86-linux-dregs.h"
#include "nat/linux-ptrace.h"
/* linux_nat_target::low_new_fork implementation. */
void
x86_linux_nat_target::low_new_fork (struct lwp_info *parent, pid_t child_pid)
{
pid_t parent_pid;
struct x86_debug_reg_state *parent_state;
struct x86_debug_reg_state *child_state;
/* NULL means no watchpoint has ever been set in the parent. In
that case, there's nothing to do. */
if (parent->arch_private == NULL)
return;
/* Linux kernel before 2.6.33 commit
72f674d203cd230426437cdcf7dd6f681dad8b0d
will inherit hardware debug registers from parent
on fork/vfork/clone. Newer Linux kernels create such tasks with
zeroed debug registers.
GDB core assumes the child inherits the watchpoints/hw
breakpoints of the parent, and will remove them all from the
forked off process. Copy the debug registers mirrors into the
new process so that all breakpoints and watchpoints can be
removed together. The debug registers mirror will become zeroed
in the end before detaching the forked off process, thus making
this compatible with older Linux kernels too. */
parent_pid = parent->ptid.pid ();
parent_state = x86_debug_reg_state (parent_pid);
child_state = x86_debug_reg_state (child_pid);
*child_state = *parent_state;
}
x86_linux_nat_target::~x86_linux_nat_target ()
{
}
void
x86_linux_nat_target::post_startup_inferior (ptid_t ptid)
{
x86_cleanup_dregs ();
linux_nat_target::post_startup_inferior (ptid);
}
#ifdef __x86_64__
/* Value of CS segment register:
64bit process: 0x33
32bit process: 0x23 */
#define AMD64_LINUX_USER64_CS 0x33
/* Value of DS segment register:
LP64 process: 0x0
X32 process: 0x2b */
#define AMD64_LINUX_X32_DS 0x2b
#endif
/* Get Linux/x86 target description from running target. */
const struct target_desc *
x86_linux_nat_target::read_description ()
{
int tid;
int is_64bit = 0;
#ifdef __x86_64__
int is_x32;
#endif
static uint64_t xcr0;
uint64_t xcr0_features_bits;
tid = inferior_ptid.pid ();
#ifdef __x86_64__
{
unsigned long cs;
unsigned long ds;
/* Get CS register. */
errno = 0;
cs = ptrace (PTRACE_PEEKUSER, tid,
offsetof (struct user_regs_struct, cs), 0);
if (errno != 0)
perror_with_name (_("Couldn't get CS register"));
is_64bit = cs == AMD64_LINUX_USER64_CS;
/* Get DS register. */
errno = 0;
ds = ptrace (PTRACE_PEEKUSER, tid,
offsetof (struct user_regs_struct, ds), 0);
if (errno != 0)
perror_with_name (_("Couldn't get DS register"));
is_x32 = ds == AMD64_LINUX_X32_DS;
if (sizeof (void *) == 4 && is_64bit && !is_x32)
error (_("Can't debug 64-bit process with 32-bit GDB"));
}
#elif HAVE_PTRACE_GETFPXREGS
if (have_ptrace_getfpxregs == -1)
{
elf_fpxregset_t fpxregs;
if (ptrace (PTRACE_GETFPXREGS, tid, 0, (int) &fpxregs) < 0)
{
have_ptrace_getfpxregs = 0;
have_ptrace_getregset = TRIBOOL_FALSE;
return i386_linux_read_description (X86_XSTATE_X87_MASK);
}
}
#endif
if (have_ptrace_getregset == TRIBOOL_UNKNOWN)
{
uint64_t xstateregs[(X86_XSTATE_SSE_SIZE / sizeof (uint64_t))];
struct iovec iov;
iov.iov_base = xstateregs;
iov.iov_len = sizeof (xstateregs);
/* Check if PTRACE_GETREGSET works. */
if (ptrace (PTRACE_GETREGSET, tid,
(unsigned int) NT_X86_XSTATE, &iov) < 0)
have_ptrace_getregset = TRIBOOL_FALSE;
else
{
have_ptrace_getregset = TRIBOOL_TRUE;
/* Get XCR0 from XSAVE extended state. */
xcr0 = xstateregs[(I386_LINUX_XSAVE_XCR0_OFFSET
/ sizeof (uint64_t))];
}
}
/* Check the native XCR0 only if PTRACE_GETREGSET is available. If
PTRACE_GETREGSET is not available then set xcr0_features_bits to
zero so that the "no-features" descriptions are returned by the
switches below. */
if (have_ptrace_getregset == TRIBOOL_TRUE)
xcr0_features_bits = xcr0 & X86_XSTATE_ALL_MASK;
else
xcr0_features_bits = 0;
if (is_64bit)
{
#ifdef __x86_64__
return amd64_linux_read_description (xcr0_features_bits, is_x32);
#endif
}
else
{
const struct target_desc * tdesc
= i386_linux_read_description (xcr0_features_bits);
if (tdesc == NULL)
tdesc = i386_linux_read_description (X86_XSTATE_SSE_MASK);
return tdesc;
}
gdb_assert_not_reached ("failed to return tdesc");
}
/* Enable branch tracing. */
struct btrace_target_info *
x86_linux_nat_target::enable_btrace (ptid_t ptid,
const struct btrace_config *conf)
{
struct btrace_target_info *tinfo = nullptr;
try
{
tinfo = linux_enable_btrace (ptid, conf);
}
catch (const gdb_exception_error &exception)
{
error (_("Could not enable branch tracing for %s: %s"),
target_pid_to_str (ptid).c_str (), exception.what ());
}
return tinfo;
}
/* Disable branch tracing. */
void
x86_linux_nat_target::disable_btrace (struct btrace_target_info *tinfo)
{
enum btrace_error errcode = linux_disable_btrace (tinfo);
if (errcode != BTRACE_ERR_NONE)
error (_("Could not disable branch tracing."));
}
/* Teardown branch tracing. */
void
x86_linux_nat_target::teardown_btrace (struct btrace_target_info *tinfo)
{
/* Ignore errors. */
linux_disable_btrace (tinfo);
}
enum btrace_error
x86_linux_nat_target::read_btrace (struct btrace_data *data,
struct btrace_target_info *btinfo,
enum btrace_read_type type)
{
return linux_read_btrace (data, btinfo, type);
}
/* See to_btrace_conf in target.h. */
const struct btrace_config *
x86_linux_nat_target::btrace_conf (const struct btrace_target_info *btinfo)
{
return linux_btrace_conf (btinfo);
}
/* Helper for ps_get_thread_area. Sets BASE_ADDR to a pointer to
the thread local storage (or its descriptor) and returns PS_OK
on success. Returns PS_ERR on failure. */
ps_err_e
x86_linux_get_thread_area (pid_t pid, void *addr, unsigned int *base_addr)
{
/* NOTE: cagney/2003-08-26: The definition of this buffer is found
in the kernel header <asm-i386/ldt.h>. It, after padding, is 4 x
4 byte integers in size: `entry_number', `base_addr', `limit',
and a bunch of status bits.
The values returned by this ptrace call should be part of the
regcache buffer, and ps_get_thread_area should channel its
request through the regcache. That way remote targets could
provide the value using the remote protocol and not this direct
call.
Is this function needed? I'm guessing that the `base' is the
address of a descriptor that libthread_db uses to find the
thread local address base that GDB needs. Perhaps that
descriptor is defined by the ABI. Anyway, given that
libthread_db calls this function without prompting (gdb
requesting tls base) I guess it needs info in there anyway. */
unsigned int desc[4];
/* This code assumes that "int" is 32 bits and that
GET_THREAD_AREA returns no more than 4 int values. */
gdb_assert (sizeof (int) == 4);
#ifndef PTRACE_GET_THREAD_AREA
#define PTRACE_GET_THREAD_AREA 25
#endif
if (ptrace (PTRACE_GET_THREAD_AREA, pid, addr, &desc) < 0)
return PS_ERR;
*base_addr = desc[1];
return PS_OK;
}
void _initialize_x86_linux_nat ();
void
_initialize_x86_linux_nat ()
{
/* Initialize the debug register function vectors. */
x86_dr_low.set_control = x86_linux_dr_set_control;
x86_dr_low.set_addr = x86_linux_dr_set_addr;
x86_dr_low.get_addr = x86_linux_dr_get_addr;
x86_dr_low.get_status = x86_linux_dr_get_status;
x86_dr_low.get_control = x86_linux_dr_get_control;
x86_set_debug_register_length (sizeof (void *));
}
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