# git rev-parse -q --verify e376ca093587eafd840bb0f9df04090e2a54249c^{commit}
e376ca093587eafd840bb0f9df04090e2a54249c
already have revision, skipping fetch
# git checkout -q -f -B kisskb e376ca093587eafd840bb0f9df04090e2a54249c
# git clean -qxdf
# < git log -1
# commit e376ca093587eafd840bb0f9df04090e2a54249c
# Author: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
# Date:   Tue Apr 21 23:05:45 2020 +0530
# 
#     powerpc/module_64: Use special stub for _mcount() with -mprofile-kernel
#     
#     Since commit c55d7b5e64265f ("powerpc: Remove STRICT_KERNEL_RWX
#     incompatibility with RELOCATABLE"), powerpc kernels with
#     -mprofile-kernel can crash in certain scenarios with a trace like below:
#     
#         BUG: Unable to handle kernel instruction fetch (NULL pointer?)
#         Faulting instruction address: 0x00000000
#         Oops: Kernel access of bad area, sig: 11 [#1]
#         LE PAGE_SIZE=64K MMU=Radix SMP NR_CPUS=256 DEBUG_PAGEALLOC NUMA PowerNV
#         <snip>
#         NIP [0000000000000000] 0x0
#         LR [c0080000102c0048] ext4_iomap_end+0x8/0x30 [ext4]
#         Call Trace:
#          iomap_apply+0x20c/0x920 (unreliable)
#          iomap_bmap+0xfc/0x160
#          ext4_bmap+0xa4/0x180 [ext4]
#          bmap+0x4c/0x80
#          jbd2_journal_init_inode+0x44/0x1a0 [jbd2]
#          ext4_load_journal+0x440/0x860 [ext4]
#          ext4_fill_super+0x342c/0x3ab0 [ext4]
#          mount_bdev+0x25c/0x290
#          ext4_mount+0x28/0x50 [ext4]
#          legacy_get_tree+0x4c/0xb0
#          vfs_get_tree+0x4c/0x130
#          do_mount+0xa18/0xc50
#          sys_mount+0x158/0x180
#          system_call+0x5c/0x68
#     
#     The NIP points to NULL, or a random location (data even), while the LR
#     always points to the LEP of a function (with an offset of 8), indicating
#     that something went wrong with ftrace. However, ftrace is not
#     necessarily active when such crashes occur.
#     
#     The kernel OOPS sometimes follows a warning from ftrace indicating that
#     some module functions could not be patched with a nop. Other times, if a
#     module is loaded early during boot, instruction patching can fail due to
#     a separate bug, but the error is not reported due to missing error
#     reporting.
#     
#     In all the above cases when instruction patching fails, ftrace will be
#     disabled but certain kernel module functions will be left with default
#     calls to _mcount(). This is not a problem with ELFv1. However, with
#     -mprofile-kernel, the default stub is problematic since it depends on a
#     valid module TOC in r2. If the kernel (or a different module) calls into
#     a function that does not use the TOC, the function won't have a prologue
#     to setup the module TOC. When that function calls into _mcount(), we
#     will end up in the relocation stub that will use the previous TOC, and
#     end up trying to jump into a random location. From the above trace:
#     
#             iomap_apply+0x20c/0x920 [kernel TOC]
#                             |
#                             V
#             ext4_iomap_end+0x8/0x30 [no GEP == kernel TOC]
#                             |
#                             V
#                     _mcount() stub
#             [uses kernel TOC -> random entry]
#     
#     To address this, let's change over to using the special stub that is
#     used for ftrace_[regs_]caller() for _mcount(). This ensures that we are
#     not dependent on a valid module TOC in r2 for default _mcount()
#     handling.
#     
#     Reported-by: Qian Cai <cai@lca.pw>
#     Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
#     Tested-by: Qian Cai <cai@lca.pw>
#     Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
#     Link: https://lore.kernel.org/r/8affd4298d22099bbd82544fab8185700a6222b1.1587488954.git.naveen.n.rao@linux.vnet.ibm.com
# < /opt/cross/kisskb/korg/gcc-5.5.0-nolibc/powerpc64-linux/bin/powerpc64-linux-gcc --version
# < /opt/cross/kisskb/korg/gcc-5.5.0-nolibc/powerpc64-linux/bin/powerpc64-linux-ld --version
# < git log --format=%s --max-count=1 e376ca093587eafd840bb0f9df04090e2a54249c
# < make -s -j 48 ARCH=powerpc O=/kisskb/build/powerpc-next_83xx_mpc837x_mds_defconfig_powerpc-gcc5 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-5.5.0-nolibc/powerpc64-linux/bin/powerpc64-linux-  83xx/mpc837x_mds_defconfig
# < make -s -j 48 ARCH=powerpc O=/kisskb/build/powerpc-next_83xx_mpc837x_mds_defconfig_powerpc-gcc5 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-5.5.0-nolibc/powerpc64-linux/bin/powerpc64-linux-  help
# make -s -j 48 ARCH=powerpc O=/kisskb/build/powerpc-next_83xx_mpc837x_mds_defconfig_powerpc-gcc5 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-5.5.0-nolibc/powerpc64-linux/bin/powerpc64-linux-  olddefconfig
# make -s -j 48 ARCH=powerpc O=/kisskb/build/powerpc-next_83xx_mpc837x_mds_defconfig_powerpc-gcc5 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-5.5.0-nolibc/powerpc64-linux/bin/powerpc64-linux-  
INFO: Uncompressed kernel (size 0x5c1f50) overlaps the address of the wrapper(0x400000)
INFO: Fixing the link_address of wrapper to (0x600000)
Image Name:   Linux-5.7.0-rc2-14860-ge376ca093
Created:      Sat May 30 06:14:12 2020
Image Type:   PowerPC Linux Kernel Image (gzip compressed)
Data Size:    2964633 Bytes = 2895.15 KiB = 2.83 MiB
Load Address: 00000000
Entry Point:  00000000
Completed OK
# rm -rf /kisskb/build/powerpc-next_83xx_mpc837x_mds_defconfig_powerpc-gcc5
# Build took: 0:00:45.942333