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The x86 mmap() code selects the mmap base for an allocation depending on
the bitness of the syscall. For 64bit sycalls it select mm->mmap_base and
for 32bit mm->mmap_compat_base.
On execve the registers of the task invoking exec() are copied to the child
pt_regs. So child->pt_regs->orig_ax contains the execve syscall number of the
parent.
exec() calls mmap() which in turn uses in_compat_syscall() to check whether
the mapping is for a 32bit or a 64bit task. The decision is made on the
following criteria:
ia32 child->thread.status & TS_COMPAT
x32 child->pt_regs.orig_ax & __X32_SYSCALL_BIT
ia64 !ia32 && !x32
child->thread.status is corretly set up in set_personality_*(), but the
syscall number in child->pt_regs.orig_ax is left unmodified.
Therefore the parent/child combinations work or fail in the following way:
Parent Child Child->thread_status child->pt_regs.orig_ax in_compat() Works
ia64 ia64 TS_COMPAT == 0 __X32_SYSCALL_BIT == 0 false Y
ia64 ia32 TS_COMPAT == 1 __X32_SYSCALL_BIT == 0 true Y
ia64 x32 TS_COMPAT == 0 __X32_SYSCALL_BIT == 0 false N
ia32 ia64 TS_COMPAT == 0 __X32_SYSCALL_BIT == 0 false Y
ia32 ia32 TS_COMPAT == 1 __X32_SYSCALL_BIT == 0 true Y
ia32 x32 TS_COMPAT == 0 __X32_SYSCALL_BIT == 0 false N
x32 ia64 TS_COMPAT == 0 __X32_SYSCALL_BIT == 1 true N
x32 ia32 TS_COMPAT == 1 __X32_SYSCALL_BIT == 1 true Y
x32 x32 TS_COMPAT == 0 __X32_SYSCALL_BIT == 1 true Y
Make set_personality_*() store the syscall number incl. __X32_SYSCALL_BIT
which corresponds to the newly started ELF executable in the childs
pt_regs, i.e. pretend that the exec was invoked from a task with the same
executable format.
So both thread.status and pt_regs.orig_ax correspond to the new ELF format
and in_compat_syscall() returns the correct result.
[ tglx: Rewrote changelog ]
Fixes: commit
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Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. 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.