e34dbbc85d
Clear the software event flag in the augmented SS to prevent immediate
repeat of single step trap on return from SIGTRAP handler if the trap
flag (TF) is set without an external debugger attached.
Following is a typical single-stepping flow for a user process:
1) The user process is prepared for single-stepping by setting
RFLAGS.TF = 1.
2) When any instruction in user space completes, a #DB is triggered.
3) The kernel handles the #DB and returns to user space, invoking the
SIGTRAP handler with RFLAGS.TF = 0.
4) After the SIGTRAP handler finishes, the user process performs a
sigreturn syscall, restoring the original state, including
RFLAGS.TF = 1.
5) Goto step 2.
According to the FRED specification:
A) Bit 17 in the augmented SS is designated as the software event
flag, which is set to 1 for FRED event delivery of SYSCALL,
SYSENTER, or INT n.
B) If bit 17 of the augmented SS is 1 and ERETU would result in
RFLAGS.TF = 1, a single-step trap will be pending upon completion
of ERETU.
In step 4) above, the software event flag is set upon the sigreturn
syscall, and its corresponding ERETU would restore RFLAGS.TF = 1.
This combination causes a pending single-step trap upon completion of
ERETU. Therefore, another #DB is triggered before any user space
instruction is executed, which leads to an infinite loop in which the
SIGTRAP handler keeps being invoked on the same user space IP.
Fixes: 14619d912b ("x86/fred: FRED entry/exit and dispatch code")
Suggested-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Signed-off-by: Xin Li (Intel) <xin@zytor.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Tested-by: Sohil Mehta <sohil.mehta@intel.com>
Cc:stable@vger.kernel.org
Link: https://lore.kernel.org/all/20250609084054.2083189-2-xin%40zytor.com
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 reStructuredText 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.