// SPDX-License-Identifier: GPL-2.0

#include <vmlinux.h>
#include <bpf/bpf_core_read.h>
#include <bpf/bpf_helpers.h>
#include <bpf/bpf_tracing.h>
#include "bpf_misc.h"

char _license[] SEC("license") = "GPL";

/* typically virtio scsi has max SGs of 6 */
#define VIRTIO_MAX_SGS  6

/* Verifier will fail with SG_MAX = 128. The failure can be
 * workarounded with a smaller SG_MAX, e.g. 10.
 */
#define WORKAROUND
#ifdef WORKAROUND
#define SG_MAX          10
#else
/* typically virtio blk has max SEG of 128 */
#define SG_MAX          128
#endif

#define SG_CHAIN        0x01UL
#define SG_END          0x02UL

#define sg_is_chain(sg)         ((sg)->page_link & SG_CHAIN)
#define sg_is_last(sg)          ((sg)->page_link & SG_END)
#define sg_chain_ptr(sg)        \
        ((struct scatterlist *) ((sg)->page_link & ~(SG_CHAIN | SG_END)))

static inline struct scatterlist *__sg_next(struct scatterlist *sgp)
{
        struct scatterlist sg;

        bpf_probe_read_kernel(&sg, sizeof(sg), sgp);
        if (sg_is_last(&sg))
                return NULL;

        sgp++;

        bpf_probe_read_kernel(&sg, sizeof(sg), sgp);
        if (sg_is_chain(&sg))
                sgp = sg_chain_ptr(&sg);

        return sgp;
}

static inline struct scatterlist *get_sgp(struct scatterlist **sgs, int i)
{
        struct scatterlist *sgp;

        bpf_probe_read_kernel(&sgp, sizeof(sgp), sgs + i);
        return sgp;
}

int run_once = 0;
int result = 0;

SEC("kprobe/virtqueue_add_sgs")
int BPF_KPROBE(trace_virtqueue_add_sgs, void *unused, struct scatterlist **sgs,
               unsigned int out_sgs, unsigned int in_sgs)
{
        struct scatterlist *sgp = NULL;
        __u64 length1 = 0, length2 = 0;
        unsigned int i, n, len;

        if (run_once != 0)
                return 0;

        for (i = 0; (i < VIRTIO_MAX_SGS) && (i < out_sgs); i++) {
                __sink(out_sgs);
                for (n = 0, sgp = get_sgp(sgs, i); sgp && (n < SG_MAX);
                     sgp = __sg_next(sgp)) {
                        len = BPF_CORE_READ(sgp, length);
                        length1 += len;
                        n++;
                }
        }

        for (i = 0; (i < VIRTIO_MAX_SGS) && (i < in_sgs); i++) {
                __sink(in_sgs);
                for (n = 0, sgp = get_sgp(sgs, i); sgp && (n < SG_MAX);
                     sgp = __sg_next(sgp)) {
                        len = BPF_CORE_READ(sgp, length);
                        length2 += len;
                        n++;
                }
        }

        run_once = 1;
        result = length2 - length1;
        return 0;
}