Message-ID: <9a35aa50-1f1d-4d50-aac7-3e0d8bbd7613@bsbernd.com>
Date: Thu, 8 Jan 2026 23:01:47 +0100
From: Bernd Schubert <bernd@...ernd.com>
To: horst@...thelmer.com, Miklos Szeredi <miklos@...redi.hu>,
 Bernd Schubert <bschubert@....com>, Joanne Koong <joannelkoong@...il.com>
Cc: linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
 Horst Birthelmer <hbirthelmer@....com>, syzbot@...kaller.appspotmail.com
Subject: Re: [PATCH RFC v3 1/3] fuse: add compound command to combine multiple
 requests



On 1/8/26 15:23, horst@...thelmer.com wrote:
> From: Horst Birthelmer <hbirthelmer@....com>
> 
> For a FUSE_COMPOUND we add a header that contains information
> about how many commands there are in the compound and about the
> size of the expected result. This will make the interpretation
> in libfuse easier, since we can preallocate the whole result.
> Then we append the requests that belong to this compound.
> 
> The API for the compound command has:
>   fuse_compound_alloc()
>   fuse_compound_add()
>   fuse_compound_send()
>   fuse_compound_free()
> 
> Signed-off-by: Horst Birthelmer <hbirthelmer@....com>
> Tested-by: syzbot@...kaller.appspotmail.com
> ---
>  fs/fuse/Makefile          |   2 +-
>  fs/fuse/compound.c        | 276 ++++++++++++++++++++++++++++++++++++++++++++++
>  fs/fuse/fuse_i.h          |  12 ++
>  include/uapi/linux/fuse.h |  37 +++++++
>  4 files changed, 326 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/fuse/Makefile b/fs/fuse/Makefile
> index 22ad9538dfc4..4c09038ef995 100644
> --- a/fs/fuse/Makefile
> +++ b/fs/fuse/Makefile
> @@ -11,7 +11,7 @@ obj-$(CONFIG_CUSE) += cuse.o
>  obj-$(CONFIG_VIRTIO_FS) += virtiofs.o
>  
>  fuse-y := trace.o	# put trace.o first so we see ftrace errors sooner
> -fuse-y += dev.o dir.o file.o inode.o control.o xattr.o acl.o readdir.o ioctl.o
> +fuse-y += dev.o dir.o file.o inode.o control.o xattr.o acl.o readdir.o ioctl.o compound.o
>  fuse-y += iomode.o
>  fuse-$(CONFIG_FUSE_DAX) += dax.o
>  fuse-$(CONFIG_FUSE_PASSTHROUGH) += passthrough.o backing.o
> diff --git a/fs/fuse/compound.c b/fs/fuse/compound.c
> new file mode 100644
> index 000000000000..2f292ae3e816
> --- /dev/null
> +++ b/fs/fuse/compound.c
> @@ -0,0 +1,276 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * FUSE: Filesystem in Userspace
> + * Copyright (C) 2025
> + *
> + * This file implements compound operations for FUSE, allowing multiple
> + * operations to be batched into a single request to reduce round trips
> + * between kernel and userspace.
> + */
> +
> +#include "fuse_i.h"
> +
> +/*
> + * Compound request builder and state tracker and args pointer storage
> + */
> +struct fuse_compound_req {
> +	struct fuse_mount *fm;
> +	struct fuse_compound_in compound_header;
> +	struct fuse_compound_out result_header;
> +
> +	/* Per-operation error codes */
> +	int op_errors[FUSE_MAX_COMPOUND_OPS];
> +	/* Original fuse_args for response parsing */
> +	struct fuse_args *op_args[FUSE_MAX_COMPOUND_OPS];
> +
> +	bool parsed;				/* Prevent double-parsing of response */

Just for Joanne and other reviewers, Horst is preparing the next
version, this 'parsed' is also going to be removed.

> +};
> +
> +struct fuse_compound_req *fuse_compound_alloc(struct fuse_mount *fm, u32 flags)
> +{
> +	struct fuse_compound_req *compound;
> +
> +	compound = kzalloc(sizeof(*compound), GFP_KERNEL);
> +	if (!compound)
> +		return ERR_PTR(-ENOMEM);
> +
> +	compound->fm = fm;
> +	compound->compound_header.flags = flags;
> +
> +	return compound;
> +}
> +
> +void fuse_compound_free(struct fuse_compound_req *compound)
> +{
> +	if (!compound)
> +		return;
> +
> +	kfree(compound);
> +}
> +
> +int fuse_compound_add(struct fuse_compound_req *compound,
> +		      struct fuse_args *args)
> +{
> +	if (!compound ||
> +	    compound->compound_header.count >= FUSE_MAX_COMPOUND_OPS)
> +		return -EINVAL;
> +
> +	if (args->in_pages)
> +		return -EINVAL;
> +
> +	compound->op_args[compound->compound_header.count] = args;
> +	compound->compound_header.count++;
> +	return 0;
> +}
> +
> +static void *fuse_copy_response_data(struct fuse_args *args,
> +				     char *response_data)
> +{
> +	size_t copied = 0;
> +	int i;
> +
> +	for (i = 0; i < args->out_numargs; i++) {
> +		struct fuse_arg current_arg = args->out_args[i];
> +		size_t arg_size;
> +
> +		/*
> +		 * Last argument with out_pages: copy to pages
> +		 * External payload (in the last out arg) is not supported
> +		 * at the moment
> +		 */
> +		if (i == args->out_numargs - 1 && args->out_pages)
> +			return response_data;
> +
> +		arg_size = current_arg.size;
> +
> +		if (current_arg.value && arg_size > 0) {
> +			memcpy(current_arg.value,
> +			       (char *)response_data + copied, arg_size);
> +			copied += arg_size;
> +		}
> +	}
> +
> +	return (char *)response_data + copied;
> +}
> +
> +int fuse_compound_get_error(struct fuse_compound_req *compound, int op_idx)
> +{
> +	return compound->op_errors[op_idx];
> +}
> +
> +static void *fuse_compound_parse_one_op(struct fuse_compound_req *compound,
> +					int op_index, void *op_out_data,
> +					void *response_end)
> +{
> +	struct fuse_out_header *op_hdr = op_out_data;
> +	struct fuse_args *args = compound->op_args[op_index];
> +
> +	if (op_hdr->len < sizeof(struct fuse_out_header))
> +		return NULL;
> +
> +	/* Check if the entire operation response fits in the buffer */
> +	if ((char *)op_out_data + op_hdr->len > (char *)response_end)
> +		return NULL;
> +
> +	if (op_hdr->error != 0)
> +		compound->op_errors[op_index] = op_hdr->error;
> +
> +	if (args && op_hdr->len > sizeof(struct fuse_out_header))
> +		return fuse_copy_response_data(args, op_out_data +
> +					       sizeof(struct fuse_out_header));
> +
> +	/* No response data, just advance past the header */
> +	return (char *)op_out_data + op_hdr->len;
> +}
> +
> +static int fuse_compound_parse_resp(struct fuse_compound_req *compound,
> +				    u32 count, void *response,
> +				    size_t response_size)
> +{
> +	void *op_out_data = response;
> +	void *response_end = (char *)response + response_size;
> +	int i;
> +
> +	if (compound->parsed)
> +		return 0;
> +
> +	if (!response || response_size < sizeof(struct fuse_out_header))
> +		return -EIO;
> +
> +	for (i = 0; i < count && i < compound->result_header.count; i++) {
> +		op_out_data = fuse_compound_parse_one_op(compound, i,
> +							 op_out_data,
> +							 response_end);
> +		if (!op_out_data)
> +			return -EIO;
> +	}
> +
> +	compound->parsed = true;
> +	return 0;
> +}
> +
> +ssize_t fuse_compound_send(struct fuse_compound_req *compound)
> +{
> +	struct fuse_args args = {
> +		.opcode = FUSE_COMPOUND,
> +		.nodeid = 0,
> +		.in_numargs = 2,
> +		.out_numargs = 2,
> +		.out_argvar = true,
> +	};
> +	size_t resp_buffer_size;
> +	size_t actual_response_size;
> +	size_t buffer_pos;
> +	size_t total_expected_out_size;
> +	void *buffer = NULL;
> +	void *resp_payload;
> +	ssize_t ret;
> +	int i;
> +
> +	if (!compound) {
> +		pr_info_ratelimited("FUSE: compound request is NULL in %s\n",
> +				    __func__);
> +		return -EINVAL;
> +	}
> +
> +	if (compound->compound_header.count == 0) {
> +		pr_info_ratelimited("FUSE: compound request contains no operations\n");
> +		return -EINVAL;
> +	}
> +
> +	buffer_pos = 0;
> +	total_expected_out_size = 0;
> +
> +	for (i = 0; i < compound->compound_header.count; i++) {
> +		struct fuse_args *op_args = compound->op_args[i];
> +		size_t needed_size = sizeof(struct fuse_in_header);
> +		int j;
> +
> +		for (j = 0; j < op_args->in_numargs; j++)
> +			needed_size += op_args->in_args[j].size;
> +
> +		buffer_pos += needed_size;
> +
> +		for (j = 0; j < op_args->out_numargs; j++)
> +			total_expected_out_size += op_args->out_args[j].size;
> +	}
> +
> +	buffer = kvmalloc(buffer_pos, GFP_KERNEL);
> +	if (!buffer)
> +		return -ENOMEM;
> +
> +	buffer_pos = 0;
> +	for (i = 0; i < compound->compound_header.count; i++) {
> +		struct fuse_args *op_args = compound->op_args[i];
> +		struct fuse_in_header *hdr;
> +		size_t needed_size = sizeof(struct fuse_in_header);
> +		int j;
> +
> +		for (j = 0; j < op_args->in_numargs; j++)
> +			needed_size += op_args->in_args[j].size;
> +
> +		hdr = (struct fuse_in_header *)(buffer + buffer_pos);
> +		memset(hdr, 0, sizeof(*hdr));
> +		hdr->len = needed_size;
> +		hdr->opcode = op_args->opcode;
> +		hdr->nodeid = op_args->nodeid;
> +		hdr->uid = from_kuid(compound->fm->fc->user_ns,
> +				     current_fsuid());
> +		hdr->gid = from_kgid(compound->fm->fc->user_ns,
> +				     current_fsgid());
> +		hdr->pid = pid_nr_ns(task_pid(current),
> +				     compound->fm->fc->pid_ns);
> +		buffer_pos += sizeof(*hdr);
> +
> +		for (j = 0; j < op_args->in_numargs; j++) {
> +			memcpy(buffer + buffer_pos, op_args->in_args[j].value,
> +			       op_args->in_args[j].size);
> +			buffer_pos += op_args->in_args[j].size;
> +		}
> +	}
> +
> +	resp_buffer_size = total_expected_out_size +
> +			   (compound->compound_header.count *
> +			    sizeof(struct fuse_out_header));
> +
> +	resp_payload = kvmalloc(resp_buffer_size, GFP_KERNEL | __GFP_ZERO);
> +	if (!resp_payload) {
> +		ret = -ENOMEM;
> +		goto out_free_buffer;
> +	}
> +
> +	compound->compound_header.result_size = total_expected_out_size;
> +
> +	args.in_args[0].size = sizeof(compound->compound_header);
> +	args.in_args[0].value = &compound->compound_header;
> +	args.in_args[1].size = buffer_pos;
> +	args.in_args[1].value = buffer;
> +
> +	args.out_args[0].size = sizeof(compound->result_header);
> +	args.out_args[0].value = &compound->result_header;
> +	args.out_args[1].size = resp_buffer_size;
> +	args.out_args[1].value = resp_payload;
> +
> +	ret = fuse_simple_request(compound->fm, &args);
> +	if (ret < 0)
> +		goto out;
> +
> +	actual_response_size = args.out_args[1].size;
> +
> +	if (actual_response_size < sizeof(struct fuse_compound_out)) {
> +		pr_info_ratelimited("FUSE: compound response too small (%zu bytes, minimum %zu bytes)\n",
> +				    actual_response_size,
> +				    sizeof(struct fuse_compound_out));
> +		ret = -EINVAL;
> +		goto out;
> +	}
> +
> +	ret = fuse_compound_parse_resp(compound, compound->result_header.count,
> +				       (char *)resp_payload,
> +				       actual_response_size);
> +out:
> +	kvfree(resp_payload);
> +out_free_buffer:
> +	kvfree(buffer);
> +	return ret;
> +}
> diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
> index 7f16049387d1..6dddbe2b027b 100644
> --- a/fs/fuse/fuse_i.h
> +++ b/fs/fuse/fuse_i.h
> @@ -1273,6 +1273,18 @@ static inline ssize_t fuse_simple_idmap_request(struct mnt_idmap *idmap,
>  int fuse_simple_background(struct fuse_mount *fm, struct fuse_args *args,
>  			   gfp_t gfp_flags);
>  
> +/**
> + * Compound request API
> + */
> +struct fuse_compound_req;
> +ssize_t fuse_compound_send(struct fuse_compound_req *compound);
> +
> +struct fuse_compound_req *fuse_compound_alloc(struct fuse_mount *fm, u32 flags);
> +int fuse_compound_add(struct fuse_compound_req *compound,
> +		      struct fuse_args *args);
> +int fuse_compound_get_error(struct fuse_compound_req *compound, int op_idx);
> +void fuse_compound_free(struct fuse_compound_req *compound);
> +
>  /**
>   * Assign a unique id to a fuse request
>   */
> diff --git a/include/uapi/linux/fuse.h b/include/uapi/linux/fuse.h
> index c13e1f9a2f12..848323acecdc 100644
> --- a/include/uapi/linux/fuse.h
> +++ b/include/uapi/linux/fuse.h
> @@ -664,6 +664,13 @@ enum fuse_opcode {
>  	FUSE_STATX		= 52,
>  	FUSE_COPY_FILE_RANGE_64	= 53,
>  
> +	/* A compound request works like multiple simple requests.
> +	 * This is a special case for calls that can be combined atomic on the
> +	 * fuse server. If the server actually does atomically execute the command is
> +	 * left to the fuse server implementation.
> +	 */
> +	FUSE_COMPOUND		= 101,

This is our internal value, next round will have set to the right value.

> +
>  	/* CUSE specific operations */
>  	CUSE_INIT		= 4096,
>  
> @@ -1245,6 +1252,36 @@ struct fuse_supp_groups {
>  	uint32_t	groups[];
>  };
>  
> +#define FUSE_MAX_COMPOUND_OPS   16        /* Maximum operations per compound */
> +
> +/*
> + * Compound request header
> + *
> + * This header is followed by the fuse requests
> + */
> +struct fuse_compound_in {
> +	uint32_t	count;			/* Number of operations */
> +	uint32_t	flags;			/* Compound flags */
> +
> +	/* Total size of all results.
> +	 * This is needed for preallocating the whole result for all
> +	 * commands in this compound.
> +	 */
> +	uint32_t	result_size;
> +	uint64_t	reserved;
> +};
> +
> +/*
> + * Compound response header
> + *
> + * This header is followed by complete fuse responses
> + */
> +struct fuse_compound_out {
> +	uint32_t	count;     /* Number of results */
> +	uint32_t	flags;     /* Result flags */
> +	uint64_t	reserved;
> +};
> +
>  /**
>   * Size of the ring buffer header
>   */
> 

Thanks,
Bernd

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