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a553476c44
emulate_step() instruction emulation including sc instruction emulation initially appeared in xmon. It was then moved into sstep.c where kprobes could use it too, and later hw_breakpoint and uprobes started to use it. Until uprobes, the only instruction emulation users were for kernel mode instructions. - xmon only steps / breaks on kernel addresses. - kprobes is kernel only. - hw_breakpoint only emulates kernel instructions, single steps user. At one point, there was support for the kernel to execute sc instructions, although that is long removed and it's not clear whether there were any in-tree users. So system call emulation is not required by the above users. uprobes uses emulate_step and it appears possible to emulate sc instruction in userspace. Userspace system call emulation is broken and it's not clear it ever worked well. The big complication is that userspace takes an interrupt to the kernel to emulate the instruction. The user->kernel interrupt sets up registers and interrupt stack frame expecting to return to userspace, then system call instruction emulation re-directs that stack frame to the kernel, early in the system call interrupt handler. This means the interrupt return code takes the kernel->kernel restore path, which does not restore everything as the system call interrupt handler would expect coming from userspace. regs->iamr appears to get lost for example, because the kernel->kernel return does not restore the user iamr. Accounting such as irqflags tracing and CPU accounting does not get flipped back to user mode as the system call handler expects, so those appear to enter the kernel twice without returning to userspace. These things may be individually fixable with various complication, but it is a big complexity for unclear real benefit. Furthermore, it is not possible to single step a system call instruction since it causes an interrupt. As such, a separate patch disables probing on system call instructions. This patch removes system call emulation and disables stepping system calls. Signed-off-by: Nicholas Piggin <npiggin@gmail.com> [minor commit log edit, and also get rid of '#ifdef CONFIG_PPC64'] Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au> Link: https://lore.kernel.org/r/a412e3b3791ed83de18704c8d90f492e7a0049c0.1648648712.git.naveen.n.rao@linux.vnet.ibm.com |
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| README | ||
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.