Re: [2/6] uaccess: add probe_kernel_write()

Ingo Molnar
2008-02-10T20:06:10Z

* Linus Torvalds <torvalds@linux-foundation.org> wrote:

> > Along Linus' suggestion to work on larger chunks in kgdb, here are
> > improved probe_kernel_read/write helpers that take a size argument.
>
> I don't think this is good.
>
> Make it
> - a function, not a #define
> - preferably uninlined (this does *not* look performance-critical)
> - get rid of the get_fs/set_fs/set_fs dance

yeah. I've done that (see the patch below), and i've just tested that
kgdb memory accesses work fine with that:

(gdb) disassemble 0xc0153ed9 0xc0153eff
Dump of assembler code from 0xc0153ed9 to 0xc0153eff:
0xc0153ed9: sfence
0xc0153edc: xchg %ax,%ax
0xc0153edf: pop %ebp
0xc0153ee0: movl $0x0,0xc0a48088
0xc0153eea: ret
0xc0153eeb: push %ebp
0xc0153eec: mov %esp,%ebp
0xc0153eee: push $0xc058f11d
0xc0153ef3: movl $0x0,0xc0a4bf4c
0xc0153efd: call 0xc0126ec5
End of assembler dump.

i have to say, it's quite nice that via kgdb i can _see_ what the
paravirt and alternatives stuff ends up patching into our binary image -
see the 'sfence' instruction above. Unfortunately looking at the vmlinux
is not as reliable as it used to be ;-)

( i've added a separate file for it under mm/maccess.c, because these
functions will be needed on NOMMU kernel too, so i couldnt move them
into their natural place, mm/memory.c. )

Ingo

---------------->
Subject: uaccess: add probe_kernel_write()
From: Ingo Molnar <mingo@elte.hu>

add probe_kernel_read() and probe_kernel_write().

Uninlined and restricted to kernel range memory only, as suggested
by Linus.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
include/linux/uaccess.h | 22 ++++++++++++++++++++++
mm/Makefile | 2 +-
mm/maccess.c | 46 ++++++++++++++++++++++++++++++++++++++++++++++
3 files changed, 69 insertions(+), 1 deletion(-)

Index: linux-kgdb.q/include/linux/uaccess.h
===================================================================
--- linux-kgdb.q.orig/include/linux/uaccess.h
+++ linux-kgdb.q/include/linux/uaccess.h
@@ -84,4 +84,26 @@ static inline unsigned long __copy_from_
ret; \
})

+/*
+ * probe_kernel_read(): safely attempt to read from a location
+ * @dst: pointer to the buffer that shall take the data
+ * @src: address to read from
+ * @size: size of the data chunk
+ *
+ * Safely read from address @src to the buffer at @dst. If a kernel fault
+ * happens, handle that and return -EFAULT.
+ */
+extern long probe_kernel_read(void *dst, void *src, size_t size);
+
+/*
+ * probe_kernel_write(): safely attempt to write to a location
+ * @dst: address to write to
+ * @src: pointer to the data that shall be written
+ * @size: size of the data chunk
+ *
+ * Safely write to address @dst from the buffer at @src. If a kernel fault
+ * happens, handle that and return -EFAULT.
+ */
+extern long probe_kernel_write(void *dst, void *src, size_t size);
+
#endif /* __LINUX_UACCESS_H__ */
Index: linux-kgdb.q/mm/Makefile
===================================================================
--- linux-kgdb.q.orig/mm/Makefile
+++ linux-kgdb.q/mm/Makefile
@@ -8,7 +8,7 @@ mmu-$(CONFIG_MMU) := fremap.o highmem.o
vmalloc.o

obj-y := bootmem.o filemap.o mempool.o oom_kill.o fadvise.o \
- page_alloc.o page-writeback.o pdflush.o \
+ maccess.o page_alloc.o page-writeback.o pdflush.o \
readahead.o swap.o truncate.o vmscan.o \
prio_tree.o util.o mmzone.o vmstat.o backing-dev.o \
page_isolation.o $(mmu-y)
Index: linux-kgdb.q/mm/maccess.c
===================================================================
--- /dev/null
+++ linux-kgdb.q/mm/maccess.c
@@ -0,0 +1,46 @@
+/*
+ * Access kernel memory without faulting.
+ */
+#include <linux/uaccess.h>
+#include <linux/mm.h>
+
+/**
+ * probe_kernel_read(): safely attempt to read from a location
+ * @dst: pointer to the buffer that shall take the data
+ * @src: address to read from
+ * @size: size of the data chunk
+ *
+ * Safely read from address @src to the buffer at @dst. If a kernel fault
+ * happens, handle that and return -EFAULT.
+ */
+long probe_kernel_read(void *dst, void *src, size_t size)
+{
+ long ret;
+
+ pagefault_disable();
+ ret = __copy_from_user_inatomic(dst,
+ (__force const void __user *)src, size);
+ pagefault_enable();
+
+ return ret ? -EFAULT : 0;
+}
+
+/**
+ * probe_kernel_write(): safely attempt to write to a location
+ * @dst: address to write to
+ * @src: pointer to the data that shall be written
+ * @size: size of the data chunk
+ *
+ * Safely write to address @dst from the buffer at @src. If a kernel fault
+ * happens, handle that and return -EFAULT.
+ */
+long probe_kernel_write(void *dst, void *src, size_t size)
+{
+ long ret;
+
+ pagefault_disable();
+ ret = __copy_to_user_inatomic((__force void __user *)dst, src, size);
+ pagefault_enable();
+
+ return ret ? -EFAULT : 0;
+}