On 9/20/21 5:25 PM, Nick Desaulniers wrote:
commit fad7cd3310db ("nbd: add the check to prevent overflow in __nbd_ioctl()") raised an issue from the fallback helpers added in commit f0907827a8a9 ("compiler.h: enable builtin overflow checkers and add fallback code")
ERROR: modpost: "__divdi3" [drivers/block/nbd.ko] undefined!
As Stephen Rothwell notes: The added check_mul_overflow() call is being passed 64 bit values. COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW is not set for this build (see include/linux/overflow.h).
Specifically, the helpers for checking whether the results of a multiplication overflowed (__unsigned_mul_overflow, __signed_add_overflow) use the division operator when !COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW. This is problematic for 64b operands on 32b hosts.
This was fixed upstream by commit 76ae847497bc ("Documentation: raise minimum supported version of GCC to 5.1") which is not suitable to be backported to stable.
Further, __builtin_mul_overflow() would emit a libcall to a compiler-rt-only symbol when compiling with clang < 14 for 32b targets.
ld.lld: error: undefined symbol: __mulodi4
In order to keep stable buildable with GCC 4.9 and clang < 14, modify struct nbd_config to instead track the number of bits of the block size; reconstructing the block size using runtime checked shifts that are not problematic for those compilers and in a ways that can be backported to stable.
In nbd_set_size, we do validate that the value of blksize must be a power of two (POT) and is in the range of [512, PAGE_SIZE] (both inclusive).
This does modify the debugfs interface.
Applied, thanks.