On Fri, 11 Sep 2020 02:05:21 +0530
Bhupesh Sharma <bhsharma@xxxxxxxxxx> wrote:
> Hi Petr,
>
> Thanks for the patch. I have some nitpicks, please see inline:
>
> On Fri, Sep 4, 2020 at 7:24 PM Petr Tesarik <ptesarik@xxxxxxx> wrote:
> >
> > Kernel commit fe557319aa06c23cffc9346000f119547e0f289a renamed
> > probe_kernel_{read,write} to copy_{from,to}_kernel_nofault.
> >
> > Additionally, commit 0493cb086353e786be56010780a0b7025b5db34c
> > unexported probe_kernel_write(), so writing kernel memory is
> > no longer possible from a module.
> >
> > I have renamed the functions in source, but I'm also adding wrappers to
> > allow building the module with older kernel versions.
> >
> > Without this patch, build with kernel 5.8 and later fails:
> >
> > kbuild/default/crash.c: In function 'crash_write':
> > kbuild/default/crash.c:189:12: error: implicit declaration of function 'probe_kernel_write'; did you mean 'kernel_write'? [-Werror=implicit-function-declaration]
> > 189 | if (probe_kernel_write(vaddr, buffer, count)) {
> > | ^~~~~~~~~~~~~~~~~~
> > | kernel_write
> > kbuild/default/crash.c: In function 'crash_read':
> > kbuild/default/crash.c:225:13: error: implicit declaration of function 'probe_kernel_read'; did you mean 'kernel_read'? [-Werror=implicit-function-declaration]
> > 225 | if (probe_kernel_read(buffer, vaddr, count)) {
> > | ^~~~~~~~~~~~~~~~~
> > | kernel_read
> >
> > Signed-off-by: Petr Tesarik <ptesarik@xxxxxxxx>
> >
> > ---
> > memory_driver/crash.c | 27 +++++++++++++++++++++++++--
> > 1 file changed, 25 insertions(+), 2 deletions(-)
> >
> > --- a/memory_driver/crash.c
> > +++ b/memory_driver/crash.c
> > @@ -25,6 +25,7 @@
> > *****************************************************************************/
> >
> > #include <linux/module.h>
> > +#include <linux/version.h>
> > #include <linux/types.h>
> > #include <linux/miscdevice.h>
> > #include <linux/init.h>
> > @@ -37,6 +38,22 @@
> >
> > extern int page_is_ram(unsigned long);
> >
> > +#if LINUX_VERSION_CODE < KERNEL_VERSION(5, 8, 0)
> > +
> > +#define CAN_WRITE_KERNEL
> 1
>
> Hmmm.. I have no strong opinion about this, but can I suggest using a
> simple variable instead of a macro.
> Something like a 'unsigned int version_info':
>
> unsigned int version_info = LINUX_VERSION_CODE;
>
> and then perform the check in the code at run time against the 'version_info'.
I'm not sure how to implement this. Kernel 5.8+ does not export any
symbol that could be used for writing, so I cannot even build the code
that needs it.
Are you suggesting to use a function pointer and initialize it using
symbol_get()?
> To me, it seems more portable in future, as we are seeing such issues
> with newer kernels (due to function name renaming/deprecation).
>
> Another minor nitpick: Can I suggest adding a comment to this code leg
> - this can help while backporting patches to distributions (which
> still work with older kernel versions) - as it helps making a better
> informed decision while backporting the patch.
OK, I get it that this would not be needed if I use symbol_get(),
correct?
Petr T
> > +
> > +static inline long copy_from_kernel_nofault(void *dst, const void
> > *src, size_t size) +{
> > + return probe_kernel_read(dst, src, size);
> > +}
> > +
> > +static inline long copy_to_kernel_nofault(void *dst, const void
> > *src, size_t size) +{
> > + return probe_kernel_write(dst, src, size);
> > +}
> > +
> > +#endif
> > +
> > #ifdef CONFIG_S390
> > /*
> > * For swapped prefix pages get bounce buffer using
> > xlate_dev_mem_ptr() @@ -160,6 +177,8 @@ crash_llseek(struct file *
> > file, loff_t }
> > }
> >
> > +#ifdef CAN_WRITE_KERNEL
> > +
> > static ssize_t
> > crash_write(struct file *file, const char *buf, size_t count,
> > loff_t *poff) {
> > @@ -186,7 +205,7 @@ crash_write(struct file *file, const cha
> > return -EFAULT;
> > }
> >
> > - if (probe_kernel_write(vaddr, buffer, count)) {
> > + if (copy_to_kernel_nofault(vaddr, buffer, count)) {
> > unmap_virtual(page);
> > return -EFAULT;
> > }
> > @@ -197,6 +216,8 @@ crash_write(struct file *file, const cha
> > return written;
> > }
> >
> > +#endif
> > +
> > /*
> > * Determine the page address for an address offset value,
> > * get a virtual address for it, and copy it out.
> > @@ -222,7 +243,7 @@ crash_read(struct file *file, char *buf,
> > * Use bounce buffer to bypass the CONFIG_HARDENED_USERCOPY
> > * kernel text restriction.
> > */
> > - if (probe_kernel_read(buffer, vaddr, count)) {
> > + if (copy_from_kernel_nofault(buffer, vaddr, count)) {
> > unmap_virtual(page);
> > return -EFAULT;
> > }
> > @@ -294,7 +315,9 @@ static struct file_operations crash_fops
> > .owner = THIS_MODULE,
> > .llseek = crash_llseek,
> > .read = crash_read,
> > +#ifdef CAN_WRITE_KERNEL
> > .write = crash_write,
> > +#endif
> > .unlocked_ioctl = crash_ioctl,
> > .open = crash_open,
> > .release = crash_release,
> > --
> > Crash-utility mailing list
> > Crash-utility@xxxxxxxxxx
> > https://www.redhat.com/mailman/listinfo/crash-utility
>
> With the above points addressed, please feel free to add:
>
> Acked-by: Bhupesh Sharma <bhsharma@xxxxxxxxxx>
>
> Thanks.
>
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