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User space access regions are tedious and require similar code patterns all
over the place:
if (!user_read_access_begin(from, sizeof(*from)))
return -EFAULT;
unsafe_get_user(val, from, Efault);
user_read_access_end();
return 0;
Efault:
user_read_access_end();
return -EFAULT;
This got worse with the recent addition of masked user access, which
optimizes the speculation prevention:
if (can_do_masked_user_access())
from = masked_user_read_access_begin((from));
else if (!user_read_access_begin(from, sizeof(*from)))
return -EFAULT;
unsafe_get_user(val, from, Efault);
user_read_access_end();
return 0;
Efault:
user_read_access_end();
return -EFAULT;
There have been issues with using the wrong user_*_access_end() variant in
the error path and other typical Copy&Pasta problems, e.g. using the wrong
fault label in the user accessor which ends up using the wrong accesss end
variant.
These patterns beg for scopes with automatic cleanup. The resulting outcome
is:
scoped_user_read_access(from, Efault)
unsafe_get_user(val, from, Efault);
return 0;
Efault:
return -EFAULT;
The scope guarantees the proper cleanup for the access mode is invoked both
in the success and the failure (fault) path.
The scoped_user_$MODE_access() macros are implemented as self terminating
nested for() loops. Thanks to Andrew Cooper for pointing me at them. The
scope can therefore be left with 'break', 'goto' and 'return'. Even
'continue' "works" due to the self termination mechanism. Both GCC and
clang optimize all the convoluted macro maze out and the above results with
clang in:
b80: f3 0f 1e fa endbr64
b84: 48 b8 ef cd ab 89 67 45 23 01 movabs $0x123456789abcdef,%rax
b8e: 48 39 c7 cmp %rax,%rdi
b91: 48 0f 47 f8 cmova %rax,%rdi
b95: 90 nop
b96: 90 nop
b97: 90 nop
b98: 31 c9 xor %ecx,%ecx
b9a: 8b 07 mov (%rdi),%eax
b9c: 89 06 mov %eax,(%rsi)
b9e: 85 c9 test %ecx,%ecx
ba0: 0f 94 c0 sete %al
ba3: 90 nop
ba4: 90 nop
ba5: 90 nop
ba6: c3 ret
Which looks as compact as it gets. The NOPs are placeholder for STAC/CLAC.
GCC emits the fault path seperately:
bf0: f3 0f 1e fa endbr64
bf4: 48 b8 ef cd ab 89 67 45 23 01 movabs $0x123456789abcdef,%rax
bfe: 48 39 c7 cmp %rax,%rdi
c01: 48 0f 47 f8 cmova %rax,%rdi
c05: 90 nop
c06: 90 nop
c07: 90 nop
c08: 31 d2 xor %edx,%edx
c0a: 8b 07 mov (%rdi),%eax
c0c: 89 06 mov %eax,(%rsi)
c0e: 85 d2 test %edx,%edx
c10: 75 09 jne c1b <afoo+0x2b>
c12: 90 nop
c13: 90 nop
c14: 90 nop
c15: b8 01 00 00 00 mov $0x1,%eax
c1a: c3 ret
c1b: 90 nop
c1c: 90 nop
c1d: 90 nop
c1e: 31 c0 xor %eax,%eax
c20: c3 ret
The fault labels for the scoped*() macros and the fault labels for the
actual user space accessors can be shared and must be placed outside of the
scope.
If masked user access is enabled on an architecture, then the pointer
handed in to scoped_user_$MODE_access() can be modified to point to a
guaranteed faulting user address. This modification is only scope local as
the pointer is aliased inside the scope. When the scope is left the alias
is not longer in effect. IOW the original pointer value is preserved so it
can be used e.g. for fixup or diagnostic purposes in the fault path.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://patch.msgid.link/20251027083745.546420421@linutronix.de
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.