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[176.36.0.241]) by smtp.gmail.com with ESMTPSA id u18-20020aa7db92000000b00554d6b46a3dsm13061433edt.46.2024.01.03.12.57.34 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 03 Jan 2024 12:57:34 -0800 (PST) Message-ID: Subject: Re: [PATCH v2 bpf-next 5/9] libbpf: use stable map placeholder FDs From: Eduard Zingerman To: Andrii Nakryiko , bpf@vger.kernel.org, ast@kernel.org, daniel@iogearbox.net, martin.lau@kernel.org Cc: kernel-team@meta.com, Jiri Olsa Date: Wed, 03 Jan 2024 22:57:33 +0200 In-Reply-To: <20240102190055.1602698-6-andrii@kernel.org> References: <20240102190055.1602698-1-andrii@kernel.org> <20240102190055.1602698-6-andrii@kernel.org> Autocrypt: addr=eddyz87@gmail.com; prefer-encrypt=mutual; keydata=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 nYzhLWGcczc6J71q1Dje0l5vIPaSFOgwmWD4DA+WvuxM/shH4rtWeodbv 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 t1iq+gsfnXbPz5AnS598ScZI1oP7OrPSFJkt/z4acEbOQDQs8aUqrd46PV jsdqGvKnXZxzylux29UTNby4jTlz9pNJM+wPrDRmGfchLDUmf6CffaUYCbu4FiId+9+dcTCDvxbABRy1C3OJ8QY7cxfJ+pEZW18fRJ0XCl/fiV/ecAOfB3HsqgTzAn555h0rkFgay0hAvMU/mAW/CFNSIxV397zm749ZNLA0L2dMy1AKuOqH+/B+/ImBfJMDjmdyJQ8WU/OFRuGLdqOd2oZrA1iuPIa+yUYyZkaZfz/emQwpIL1+Q4p1R/OplA4yc301AqruXXUcVDbEB+joHW3hy5FwK5t5OwTKatrSJBkydSF9zdXy98fYzGniRyRA65P0Ix/8J3BYB4edY2/w0Ip/mdYsYQljBY0A== Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.50.2 Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Tbh, it looks like calls to zclose(map->fd) were unnecessary regardless of this patch, as all maps are closed at the end of bpf_object_load() in case of an error. [...] > diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c > index f29cfb344f80..e0085aef17d7 100644 > --- a/tools/lib/bpf/libbpf.c > +++ b/tools/lib/bpf/libbpf.c [...] > @@ -5275,13 +5289,11 @@ static int bpf_object_create_map(struct bpf_objec= t *obj, struct bpf_map *map, bo > create_attr.btf_value_type_id =3D 0; > map->btf_key_type_id =3D 0; > map->btf_value_type_id =3D 0; > - map->fd =3D bpf_map_create(def->type, map_name, > - def->key_size, def->value_size, > - def->max_entries, &create_attr); > + map_fd =3D bpf_map_create(def->type, map_name, > + def->key_size, def->value_size, > + def->max_entries, &create_attr); > } > =20 > - err =3D map->fd < 0 ? -errno : 0; > - > if (bpf_map_type_is_map_in_map(def->type) && map->inner_map) { > if (obj->gen_loader) > map->inner_map->fd =3D -1; > @@ -5289,7 +5301,19 @@ static int bpf_object_create_map(struct bpf_object= *obj, struct bpf_map *map, bo > zfree(&map->inner_map); > } > =20 > - return err; > + if (map_fd < 0) > + return -errno; Nit: this check is now placed after call to bpf_map_destroy(), which might call munmap(), which might overwrite "errno", set by some of the previous calls to bpf_map_create(). [...] > diff --git a/tools/lib/bpf/libbpf_internal.h b/tools/lib/bpf/libbpf_inter= nal.h > index b5d334754e5d..662a3df1e29f 100644 > --- a/tools/lib/bpf/libbpf_internal.h > +++ b/tools/lib/bpf/libbpf_internal.h > @@ -555,6 +555,30 @@ static inline int ensure_good_fd(int fd) > return fd; > } > =20 > +static inline int create_placeholder_fd(void) > +{ > + int fd; > + > + fd =3D ensure_good_fd(open("/dev/null", O_WRONLY | O_CLOEXEC)); Stupid question: is it ok to assume that /dev is always mounted? Googling says that kernel chooses if to mount it automatically depending on the value of CONFIG_DEVTMPFS_MOUNT option. Another option might be memfd_create(). > + if (fd < 0) > + return -errno; > + return fd; > +} [...]