> On Oct 17, 2023, at 11:58 AM, Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Fri, Oct 13, 2023 at 11:29 AM Song Liu <song@xxxxxxxxxx> wrote:
>>
>> This kfunc can be used to read xattr of a file.
>>
>> Since vfs_getxattr() requires null-terminated string as input "name", a new
>> helper bpf_dynptr_is_string() is added to check the input before calling
>> vfs_getxattr().
>>
>> Signed-off-by: Song Liu <song@xxxxxxxxxx>
>> ---
>> include/linux/bpf.h | 12 +++++++++++
>> kernel/trace/bpf_trace.c | 44 ++++++++++++++++++++++++++++++++++++++++
>> 2 files changed, 56 insertions(+)
>>
>> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
>> index 61bde4520f5c..f14fae45e13d 100644
>> --- a/include/linux/bpf.h
>> +++ b/include/linux/bpf.h
>> @@ -2472,6 +2472,13 @@ static inline bool has_current_bpf_ctx(void)
>> return !!current->bpf_ctx;
>> }
>>
>> +static inline bool bpf_dynptr_is_string(struct bpf_dynptr_kern *ptr)
>
> is_zero_terminated would be more accurate? though there is nothing
> really dynptr-specific here...
is_zero_terminated sounds better.
>
>> +{
>> + char *str = ptr->data;
>> +
>> + return str[__bpf_dynptr_size(ptr) - 1] == '\0';
>> +}
>> +
>> void notrace bpf_prog_inc_misses_counter(struct bpf_prog *prog);
>>
>> void bpf_dynptr_init(struct bpf_dynptr_kern *ptr, void *data,
>> @@ -2708,6 +2715,11 @@ static inline bool has_current_bpf_ctx(void)
>> return false;
>> }
>>
>> +static inline bool bpf_dynptr_is_string(struct bpf_dynptr_kern *ptr)
>> +{
>> + return false;
>> +}
>> +
>> static inline void bpf_prog_inc_misses_counter(struct bpf_prog *prog)
>> {
>> }
>> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
>> index df697c74d519..946268574e05 100644
>> --- a/kernel/trace/bpf_trace.c
>> +++ b/kernel/trace/bpf_trace.c
>> @@ -24,6 +24,7 @@
>> #include <linux/key.h>
>> #include <linux/verification.h>
>> #include <linux/namei.h>
>> +#include <linux/fileattr.h>
>>
>> #include <net/bpf_sk_storage.h>
>>
>> @@ -1429,6 +1430,49 @@ static int __init bpf_key_sig_kfuncs_init(void)
>> late_initcall(bpf_key_sig_kfuncs_init);
>> #endif /* CONFIG_KEYS */
>>
>> +/* filesystem kfuncs */
>> +__diag_push();
>> +__diag_ignore_all("-Wmissing-prototypes",
>> + "kfuncs which will be used in BPF programs");
>> +
>> +/**
>> + * bpf_get_file_xattr - get xattr of a file
>> + * @name_ptr: name of the xattr
>> + * @value_ptr: output buffer of the xattr value
>> + *
>> + * Get xattr *name_ptr* of *file* and store the output in *value_ptr*.
>> + *
>> + * Return: 0 on success, a negative value on error.
>> + */
>> +__bpf_kfunc int bpf_get_file_xattr(struct file *file, struct bpf_dynptr_kern *name_ptr,
>> + struct bpf_dynptr_kern *value_ptr)
>> +{
>> + if (!bpf_dynptr_is_string(name_ptr))
>> + return -EINVAL;
>
> so dynptr can be invalid and name_ptr->data will be NULL, you should
> account for that
We can add a NULL check (or size check) here.
>
> and there could also be special dynptrs that don't have contiguous
> memory region, so somehow you'd need to take care of that as well
We can require the dynptr to be BPF_DYNPTR_TYPE_LOCAL. I don't think
we need this for dynptr of skb or xdp. Would this be sufficient?
Thanks,
Song
>
>> +
>> + return vfs_getxattr(mnt_idmap(file->f_path.mnt), file_dentry(file), name_ptr->data,
>> + value_ptr->data, __bpf_dynptr_size(value_ptr));
>> +}
