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linux-user/sh4: Fix crashes on signal delivery 

sh4 uses gUSA (general UserSpace Atomicity) to provide atomicity on CPUs
that don't have atomic instructions. A gUSA region that adds 1 to an
atomic variable stored in @R2 looks like this:

  4004b6:       03 c7           mova    4004c4 <gusa+0x10>,r0
  4004b8:       f3 61           mov     r15,r1
  4004ba:       09 00           nop
  4004bc:       fa ef           mov     #-6,r15
  4004be:       22 63           mov.l   @r2,r3
  4004c0:       01 73           add     #1,r3
  4004c2:       32 22           mov.l   r3,@r2
  4004c4:       13 6f           mov     r1,r15

R0 contains a pointer to the end of the gUSA region
R1 contains the saved stack pointer
R15 contains negative length of the gUSA region

When this region is interrupted by a signal, the kernel detects if
R15 >= -128U. If yes, the kernel rolls back PC to the beginning of the
region and restores SP by copying R1 to R15.

The problem happens if we are interrupted by a signal at address 4004c4.
R15 still holds the value -6, but the atomic value was already written by
an instruction at address 4004c2. In this situation we can't undo the
gUSA. The function unwind_gusa does nothing, the signal handler attempts
to push a signal frame to the address -6 and crashes.

This patch fixes it, so that if we are interrupted at the last instruction
in a gUSA region, we copy R1 to R15 to restore the correct stack pointer
and avoid crashing.

There's another bug: if we are interrupted in a delay slot, we save the
address of the instruction in the delay slot. We must save the address of
the previous instruction.

Cc: qemu-stable@nongnu.org
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Reviewed-by: Yoshinori Sato <ysato@users.sourcefoege.jp>
Message-Id: <b16389f7-6c62-70b7-59b3-87533c0bcc@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
(cherry picked from commit 3b894b699c)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
This commit is contained in:
Mikulas Patocka 2023-09-28 18:42:08 +02:00 committed by Michael Tokarev
parent 814f91d679
commit 1e4c468ec7
1 changed files with 8 additions and 0 deletions
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8
linux-user/sh4/signal.c
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@ -104,6 +104,14 @@ static void unwind_gusa(CPUSH4State *regs)
/* Reset the SP to the saved version in R1. */
regs->gregs[15] = regs->gregs[1];
} else if (regs->gregs[15] >= -128u && regs->pc == regs->gregs[0]) {
/* If we are on the last instruction of a gUSA region, we must reset
the SP, otherwise we would be pushing the signal context to
invalid memory. */
regs->gregs[15] = regs->gregs[1];
} else if (regs->flags & TB_FLAG_DELAY_SLOT) {
/* If we are in a delay slot, push the previous instruction. */
regs->pc -= 2;
}
}