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5ee506ed35
A not so recent kernel change[1] changed how the trampoline
`__kernel_sigtramp_rt64` is used to call signal handlers.
This was exposed on the test misc/tst-sigcontext-get_pc
Before kernel 5.9, the kernel set LR to the trampoline address and
jumped directly to the signal handler, and at the end the signal
handler, as any other function, would `blr` to the address set. In
other words, the trampoline was executed just at the end of the signal
handler and the only thing it did was call sigreturn. But since
kernel 5.9 the kernel set CTRL to the signal handler and calls to the
trampoline code, the trampoline then `bctrl` to the address in CTRL,
setting the LR to the next instruction in the middle of the
trampoline, when the signal handler returns, the rest of the
trampoline code executes the same code as before.
Here is the full trampoline code as of kernel 5.11.0-rc5 for
reference:
V_FUNCTION_BEGIN(__kernel_sigtramp_rt64)
.Lsigrt_start:
bctrl /* call the handler */
addi r1, r1, __SIGNAL_FRAMESIZE
li r0,__NR_rt_sigreturn
sc
.Lsigrt_end:
V_FUNCTION_END(__kernel_sigtramp_rt64)
This new behavior breaks how `backtrace()` uses to detect the
trampoline frame to correctly reconstruct the stack frame when it is
called from inside a signal handling.
This workaround rely on the fact that the trampoline code is at very
least two (maybe 3?) instructions in size (as it is in the 32 bits
version, only on `li` and `sc`), so it is safe to check the return
address be in the range __kernel_sigtramp_rt64 .. + 4.
[1] subject: powerpc/64/signal: Balance return predictor stack in signal trampoline
commit: 0138ba5783ae0dcc799ad401a1e8ac8333790df9
url: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=0138ba5783ae0dcc799ad401a1e8ac8333790df9
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
118 lines
4.0 KiB
C
118 lines
4.0 KiB
C
/* Return backtrace of current program state.
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Copyright (C) 1998-2021 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Library General Public License as
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published by the Free Software Foundation; either version 2 of the
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License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Library General Public License for more details.
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You should have received a copy of the GNU Library General Public
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License along with the GNU C Library; see the file COPYING.LIB. If
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not, see <https://www.gnu.org/licenses/>. */
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#include <stddef.h>
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#include <string.h>
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#include <signal.h>
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#include <stdint.h>
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#include <execinfo.h>
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#include <libc-vdso.h>
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/* This is the stack layout we see with every stack frame.
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Note that every routine is required by the ABI to lay out the stack
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like this.
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+----------------+ +-----------------+
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%r1 -> | %r1 last frame--------> | %r1 last frame--->... --> NULL
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| | | |
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| cr save | | cr save |
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| | | |
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| (unused) | | return address |
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+----------------+ +-----------------+
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*/
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struct layout
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{
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struct layout *next;
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long int condition_register;
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void *return_address;
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};
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/* Since the signal handler is just like any other function it needs to
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save/restore its LR and it will save it into callers stack frame.
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Since a signal handler doesn't have a caller, the kernel creates a
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dummy frame to make it look like it has a caller. */
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struct signal_frame_64 {
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#define SIGNAL_FRAMESIZE 128
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char dummy[SIGNAL_FRAMESIZE];
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ucontext_t uc;
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/* We don't care about the rest, since the IP value is at 'uc' field. */
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};
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/* Test if the address match to the inside the trampoline code.
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Up to and including kernel 5.8, returning from an interrupt or syscall to a
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signal handler starts execution directly at the handler's entry point, with
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LR set to address of the sigreturn trampoline (the vDSO symbol).
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Newer kernels will branch to signal handler from the trampoline instead, so
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checking the stacktrace against the vDSO entrypoint does not work in such
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case.
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The vDSO branches with a 'bctrl' instruction, so checking either the
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vDSO address itself and the next instruction should cover all kernel
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versions. */
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static inline bool
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is_sigtramp_address (void *nip)
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{
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#ifdef HAVE_SIGTRAMP_RT64
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if (nip == GLRO (dl_vdso_sigtramp_rt64) ||
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nip == GLRO (dl_vdso_sigtramp_rt64) + 4)
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return true;
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#endif
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return false;
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}
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int
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__backtrace (void **array, int size)
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{
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struct layout *current;
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int count;
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/* Force gcc to spill LR. */
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asm volatile ("" : "=l"(current));
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/* Get the address on top-of-stack. */
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asm volatile ("ld %0,0(1)" : "=r"(current));
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for ( count = 0;
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current != NULL && count < size;
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current = current->next, count++)
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{
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array[count] = current->return_address;
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/* Check if the symbol is the signal trampoline and get the interrupted
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* symbol address from the trampoline saved area. */
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if (is_sigtramp_address (current->return_address))
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{
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struct signal_frame_64 *sigframe = (struct signal_frame_64*) current;
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if (count + 1 == size)
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break;
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array[++count] = (void*) sigframe->uc.uc_mcontext.gp_regs[PT_NIP];
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current = (void*) sigframe->uc.uc_mcontext.gp_regs[PT_R1];
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}
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}
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/* It's possible the second-last stack frame can't return
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(that is, it's __libc_start_main), in which case
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the CRT startup code will have set its LR to 'NULL'. */
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if (count > 0 && array[count-1] == NULL)
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count--;
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return count;
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}
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weak_alias (__backtrace, backtrace)
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libc_hidden_def (__backtrace)
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