Some examples for inject inject guarantees the appropriate erroneous return of the specified injection mode (kmalloc,bio,etc) given a call chain and an optional set of predicates. You can also optionally print out the generated BPF program for modification/debugging purposes. As a simple example, let's say you wanted to fail all mounts. As of 4.17 we can fail syscalls directly, so let's do that: # ./inject.py kmalloc -v 'SyS_mount()' The first argument indicates the mode (or what to fail). Appropriate headers are specified, if necessary. The verbosity flag prints the generated program. Note that some syscalls will be available as 'SyS_xyz' and some will be available as 'sys_xyz'. This is largely dependent on the number of arguments each syscall takes. Trying to mount various filesystems will fail and report an inability to allocate memory, as expected. Whenever a predicate is missing, an implicit "(true)" is inserted. The example above can be explicitly written as: # ./inject.py kmalloc -v '(true) => SyS_mount()(true)' The "(true)" without an associated function is a predicate for the error injection mechanism of the current mode. In the case of kmalloc, the predicate would have access to the arguments of: int should_failslab(struct kmem_cache *s, gfp_t gfpflags); The bio mode works similarly, with access to the arguments of: static noinline int should_fail_bio(struct bio *bio) We also note that it's unnecessary to state the arguments of the function if you have no intention to reference them in the associated predicate. Now let's say we want to be a bit more specific; suppose you want to fail kmalloc() from mount_subtree() when called from btrfs_mount(). This will fail only btrfs mounts: # ./inject.py kmalloc -v 'mount_subtree() => btrfs_mount()' Attempting to mount btrfs filesystem during the execution of this command will yield an error, but other filesystems will be fine. Next, lets say we want to hit one of the BUG_ONs in fs/btrfs. As of 4.16-rc3, there is a BUG_ON in btrfs_prepare_close_one_device() at fs/btrfs/volumes.c:1002 To hit this, we can use the following: # ./inject.py kmalloc -v 'btrfs_alloc_device() => btrfs_close_devices()' While the script was executing, I mounted and unmounted btrfs, causing a segfault on umount(since that satisfied the call path indicated). A look at dmesg will confirm that the erroneous return value injected by the script tripped the BUG_ON, causing a segfault down the line. In general, it's worth noting that the required specificity of the call chain is dependent on how much granularity you need. The example above might have performed as expected without the intermediate btrfs_alloc_device, but might have also done something unexpected(an earlier kmalloc could have failed before the one we were targetting). For hot paths, the approach outlined above isn't enough. If a path is traversed very often, we can distinguish distinct calls with function arguments. Let's say we want to fail the dentry allocation of a file creatively named 'bananas'. We can do the following: # ./inject.py kmalloc -v 'd_alloc_parallel(struct dentry *parent, const struct qstr *name)(STRCMP(name->name, 'bananas'))' While this script is executing, any operation that would cause a dentry allocation where the name is 'bananas' fails, as expected. Here, since we're referencing a function argument in our predicate, we need to provide the function signature up to the argument we're using. To note, STRCMP is a workaround for some rewriter issues. It will take input of the form (x->...->z, 'literal'), and generate some equivalent code that the verifier is more friendly about. It's not horribly robust, but works for the purposes of making string comparisons a bit easier. Finally, we briefly demonstrate how to inject bio failures. The mechanism is identical, so any information from above will apply. Let's say we want to fail bio requests when the request is to some specific sector. An example use case would be to fail superblock writes in btrfs. For btrfs, we know that there must be a superblock at 65536 bytes, or sector 128. This allows us to run the following: # ./inject.py bio -v -I 'linux/blkdev.h' '(({struct gendisk *d = bio->bi_disk; struct disk_part_tbl *tbl = d->part_tbl; struct hd_struct **parts = (void *)tbl + sizeof(struct disk_part_tbl); struct hd_struct **partp = parts + bio->bi_partno; struct hd_struct *p = *partp; dev_t disk = p->__dev.devt; disk == MKDEV(254,16);}) && bio->bi_iter.bi_sector == 128)' The predicate in the command above has two parts. The first is a compound statement which shortens to "only if the system is btrfs", but is long due to rewriter/verifier shenanigans. The major/minor information can be found however; I used Python. The second part simply checks the starting address of bi_iter. While executing, this script effectively fails superblock writes to the superblock at sector 128 without affecting other filesystems. As an extension to the above, one could easily fail all btrfs superblock writes (we only fail the primary) by calculating the sector number of the mirrors and amending the predicate accordingly. Inject also provides a probability option; this allows you to fail the path+predicates some percentage of the time. For example, let's say we want to fail our mounts half the time: # ./inject.py kmalloc -v -P 0.01 'SyS_mount()' USAGE message: usage: inject.py [-h] [-I header] [-P probability] [-v] {kmalloc,bio} spec Fail specified kernel functionality when call chain and predicates are met positional arguments: {kmalloc,bio} indicate which base kernel function to fail spec specify call chain optional arguments: -h, --help show this help message and exit -I header, --include header additional header files to include in the BPF program -P probability, --probability probability probability that this call chain will fail -v, --verbose print BPF program EXAMPLES: # ./inject.py kmalloc -v 'SyS_mount()' Fails all calls to syscall mount # ./inject.py kmalloc -v '(true) => SyS_mount()(true)' Explicit rewriting of above # ./inject.py kmalloc -v 'mount_subtree() => btrfs_mount()' Fails btrfs mounts only # ./inject.py kmalloc -v 'd_alloc_parallel(struct dentry *parent, const struct \ qstr *name)(STRCMP(name->name, 'bananas'))' Fails dentry allocations of files named 'bananas' # ./inject.py kmalloc -v -P 0.01 'SyS_mount()' Fails calls to syscall mount with 1% probability