| Message ID | 20220628164529.80050-1-quentin@isovalent.com (mailing list archive) |
|---|---|
| State | Superseded |
| Delegated to: | BPF |
| Headers | show |
| Series | [bpf-next] bpftool: Probe for memcg-based accounting before bumping rlimit | expand |
On Tue, Jun 28, 2022 at 9:45 AM Quentin Monnet <quentin@isovalent.com> wrote: > > Bpftool used to bump the memlock rlimit to make sure to be able to load > BPF objects. After the kernel has switched to memcg-based memory > accounting [0] in 5.11, bpftool has relied on libbpf to probe the system > for memcg-based accounting support and for raising the rlimit if > necessary [1]. But this was later reverted, because the probe would > sometimes fail, resulting in bpftool not being able to load all required > objects [2]. > > Here we add a more efficient probe, in bpftool itself. We first lower > the rlimit to 0, then we attempt to load a BPF object (and finally reset > the rlimit): if the load succeeds, then memcg-based memory accounting is > supported. > > This approach was earlier proposed for the probe in libbpf itself [3], > but given that the library may be used in multithreaded applications, > the probe could have undesirable consequences if one thread attempts to > lock kernel memory while memlock rlimit is at 0. Since bpftool is > single-threaded and the rlimit is process-based, this is fine to do in > bpftool itself. > > This probe was inspired by the similar one from the cilium/ebpf Go > library [4]. > > [0] commit 97306be45fbe ("Merge branch 'switch to memcg-based memory accounting'") > [1] commit a777e18f1bcd ("bpftool: Use libbpf 1.0 API mode instead of RLIMIT_MEMLOCK") > [2] commit 6b4384ff1088 ("Revert "bpftool: Use libbpf 1.0 API mode instead of RLIMIT_MEMLOCK"") > [3] https://lore.kernel.org/bpf/20220609143614.97837-1-quentin@isovalent.com/t/#u > [4] https://github.com/cilium/ebpf/blob/v0.9.0/rlimit/rlimit.go#L39 > > Cc: Stanislav Fomichev <sdf@google.com> > Cc: Yafang Shao <laoar.shao@gmail.com> > Suggested-by: Daniel Borkmann <daniel@iogearbox.net> > Signed-off-by: Quentin Monnet <quentin@isovalent.com> > --- > tools/bpf/bpftool/common.c | 71 ++++++++++++++++++++++++++++++++++-- > tools/include/linux/kernel.h | 5 +++ > 2 files changed, 73 insertions(+), 3 deletions(-) > > diff --git a/tools/bpf/bpftool/common.c b/tools/bpf/bpftool/common.c > index a0d4acd7c54a..e07769802f76 100644 > --- a/tools/bpf/bpftool/common.c > +++ b/tools/bpf/bpftool/common.c > @@ -13,14 +13,17 @@ > #include <stdlib.h> > #include <string.h> > #include <unistd.h> > -#include <linux/limits.h> > -#include <linux/magic.h> > #include <net/if.h> > #include <sys/mount.h> > #include <sys/resource.h> > #include <sys/stat.h> > #include <sys/vfs.h> > > +#include <linux/filter.h> > +#include <linux/limits.h> > +#include <linux/magic.h> > +#include <linux/unistd.h> > + > #include <bpf/bpf.h> > #include <bpf/hashmap.h> > #include <bpf/libbpf.h> /* libbpf_num_possible_cpus */ > @@ -73,11 +76,73 @@ static bool is_bpffs(char *path) > return (unsigned long)st_fs.f_type == BPF_FS_MAGIC; > } > > +/* Probe whether kernel switched from memlock-based (RLIMIT_MEMLOCK) to > + * memcg-based memory accounting for BPF maps and programs. This was done in > + * commit 97306be45fbe ("Merge branch 'switch to memcg-based memory > + * accounting'"), in Linux 5.11. > + * > + * Libbpf also offers to probe for memcg-based accounting vs rlimit, but does > + * so by checking for the availability of a given BPF helper and this has > + * failed on some kernels with backports in the past, see commit 6b4384ff1088 > + * ("Revert "bpftool: Use libbpf 1.0 API mode instead of RLIMIT_MEMLOCK""). > + * Instead, we can probe by lowering the process-based rlimit to 0, trying to > + * load a BPF object, and resetting the rlimit. If the load succeeds then > + * memcg-based accounting is supported. > + * > + * This would be too dangerous to do in the library, because multithreaded > + * applications might attempt to load items while the rlimit is at 0. Given > + * that bpftool is single-threaded, this is fine to do here. > + */ > +static bool known_to_need_rlimit(void) > +{ > + const size_t prog_load_attr_sz = offsetofend(union bpf_attr, attach_btf_obj_fd); nit: Any specific reason you're hard coding this sz via offseofend? Why not use sizeof(bpf_attr) directly as a syscall/memset size? The kernel should handle all these cases where bpftool has extra zero padding, right? > + struct bpf_insn insns[] = { > + BPF_EXIT_INSN(), > + }; > + struct rlimit rlim_init, rlim_cur_zero = {}; > + size_t insn_cnt = ARRAY_SIZE(insns); > + union bpf_attr attr; > + int prog_fd, err; > + > + memset(&attr, 0, prog_load_attr_sz); > + attr.prog_type = BPF_PROG_TYPE_SOCKET_FILTER; > + attr.insns = ptr_to_u64(insns); > + attr.insn_cnt = insn_cnt; > + attr.license = ptr_to_u64("GPL"); > + > + if (getrlimit(RLIMIT_MEMLOCK, &rlim_init)) > + return false; > + > + /* Drop the soft limit to zero. We maintain the hard limit to its > + * current value, because lowering it would be a permanent operation > + * for unprivileged users. > + */ > + rlim_cur_zero.rlim_max = rlim_init.rlim_max; > + if (setrlimit(RLIMIT_MEMLOCK, &rlim_cur_zero)) > + return false; > + > + /* Do not use bpf_prog_load() from libbpf here, because it calls > + * bump_rlimit_memlock(), interfering with the current probe. > + */ > + prog_fd = syscall(__NR_bpf, BPF_PROG_LOAD, &attr, prog_load_attr_sz); > + err = errno; > + > + /* reset soft rlimit to its initial value */ > + setrlimit(RLIMIT_MEMLOCK, &rlim_init); > + > + if (prog_fd < 0) > + return err == EPERM; > + > + close(prog_fd); > + return false; > +} > + > void set_max_rlimit(void) > { > struct rlimit rinf = { RLIM_INFINITY, RLIM_INFINITY }; > > - setrlimit(RLIMIT_MEMLOCK, &rinf); > + if (known_to_need_rlimit()) > + setrlimit(RLIMIT_MEMLOCK, &rinf); > } > > static int > diff --git a/tools/include/linux/kernel.h b/tools/include/linux/kernel.h > index 4b0673bf52c2..5c90d65cc2d3 100644 > --- a/tools/include/linux/kernel.h > +++ b/tools/include/linux/kernel.h > @@ -24,6 +24,11 @@ > #define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER) > #endif > > +#ifndef offsetofend > +# define offsetofend(TYPE, FIELD) \ > + (offsetof(TYPE, FIELD) + sizeof(((TYPE *)0)->FIELD)) > +#endif > + > #ifndef container_of > /** > * container_of - cast a member of a structure out to the containing structure > -- > 2.34.1 >
On 28/06/2022 18:53, Stanislav Fomichev wrote: > On Tue, Jun 28, 2022 at 9:45 AM Quentin Monnet <quentin@isovalent.com> wrote: >> >> Bpftool used to bump the memlock rlimit to make sure to be able to load >> BPF objects. After the kernel has switched to memcg-based memory >> accounting [0] in 5.11, bpftool has relied on libbpf to probe the system >> for memcg-based accounting support and for raising the rlimit if >> necessary [1]. But this was later reverted, because the probe would >> sometimes fail, resulting in bpftool not being able to load all required >> objects [2]. >> >> Here we add a more efficient probe, in bpftool itself. We first lower >> the rlimit to 0, then we attempt to load a BPF object (and finally reset >> the rlimit): if the load succeeds, then memcg-based memory accounting is >> supported. >> >> This approach was earlier proposed for the probe in libbpf itself [3], >> but given that the library may be used in multithreaded applications, >> the probe could have undesirable consequences if one thread attempts to >> lock kernel memory while memlock rlimit is at 0. Since bpftool is >> single-threaded and the rlimit is process-based, this is fine to do in >> bpftool itself. >> >> This probe was inspired by the similar one from the cilium/ebpf Go >> library [4]. >> >> [0] commit 97306be45fbe ("Merge branch 'switch to memcg-based memory accounting'") >> [1] commit a777e18f1bcd ("bpftool: Use libbpf 1.0 API mode instead of RLIMIT_MEMLOCK") >> [2] commit 6b4384ff1088 ("Revert "bpftool: Use libbpf 1.0 API mode instead of RLIMIT_MEMLOCK"") >> [3] https://lore.kernel.org/bpf/20220609143614.97837-1-quentin@isovalent.com/t/#u >> [4] https://github.com/cilium/ebpf/blob/v0.9.0/rlimit/rlimit.go#L39 >> >> Cc: Stanislav Fomichev <sdf@google.com> >> Cc: Yafang Shao <laoar.shao@gmail.com> >> Suggested-by: Daniel Borkmann <daniel@iogearbox.net> >> Signed-off-by: Quentin Monnet <quentin@isovalent.com> >> --- >> tools/bpf/bpftool/common.c | 71 ++++++++++++++++++++++++++++++++++-- >> tools/include/linux/kernel.h | 5 +++ >> 2 files changed, 73 insertions(+), 3 deletions(-) >> >> diff --git a/tools/bpf/bpftool/common.c b/tools/bpf/bpftool/common.c >> index a0d4acd7c54a..e07769802f76 100644 >> --- a/tools/bpf/bpftool/common.c >> +++ b/tools/bpf/bpftool/common.c >> @@ -13,14 +13,17 @@ >> #include <stdlib.h> >> #include <string.h> >> #include <unistd.h> >> -#include <linux/limits.h> >> -#include <linux/magic.h> >> #include <net/if.h> >> #include <sys/mount.h> >> #include <sys/resource.h> >> #include <sys/stat.h> >> #include <sys/vfs.h> >> >> +#include <linux/filter.h> >> +#include <linux/limits.h> >> +#include <linux/magic.h> >> +#include <linux/unistd.h> >> + >> #include <bpf/bpf.h> >> #include <bpf/hashmap.h> >> #include <bpf/libbpf.h> /* libbpf_num_possible_cpus */ >> @@ -73,11 +76,73 @@ static bool is_bpffs(char *path) >> return (unsigned long)st_fs.f_type == BPF_FS_MAGIC; >> } >> >> +/* Probe whether kernel switched from memlock-based (RLIMIT_MEMLOCK) to >> + * memcg-based memory accounting for BPF maps and programs. This was done in >> + * commit 97306be45fbe ("Merge branch 'switch to memcg-based memory >> + * accounting'"), in Linux 5.11. >> + * >> + * Libbpf also offers to probe for memcg-based accounting vs rlimit, but does >> + * so by checking for the availability of a given BPF helper and this has >> + * failed on some kernels with backports in the past, see commit 6b4384ff1088 >> + * ("Revert "bpftool: Use libbpf 1.0 API mode instead of RLIMIT_MEMLOCK""). >> + * Instead, we can probe by lowering the process-based rlimit to 0, trying to >> + * load a BPF object, and resetting the rlimit. If the load succeeds then >> + * memcg-based accounting is supported. >> + * >> + * This would be too dangerous to do in the library, because multithreaded >> + * applications might attempt to load items while the rlimit is at 0. Given >> + * that bpftool is single-threaded, this is fine to do here. >> + */ >> +static bool known_to_need_rlimit(void) >> +{ >> + const size_t prog_load_attr_sz = offsetofend(union bpf_attr, attach_btf_obj_fd); > > nit: > Any specific reason you're hard coding this sz via offseofend? Why not > use sizeof(bpf_attr) directly as a syscall/memset size? > The kernel should handle all these cases where bpftool has extra zero > padding, right? No particular reason. Good point, I'll send a v2 to address this. Thanks, Quentin
diff --git a/tools/bpf/bpftool/common.c b/tools/bpf/bpftool/common.c index a0d4acd7c54a..e07769802f76 100644 --- a/tools/bpf/bpftool/common.c +++ b/tools/bpf/bpftool/common.c @@ -13,14 +13,17 @@ #include <stdlib.h> #include <string.h> #include <unistd.h> -#include <linux/limits.h> -#include <linux/magic.h> #include <net/if.h> #include <sys/mount.h> #include <sys/resource.h> #include <sys/stat.h> #include <sys/vfs.h> +#include <linux/filter.h> +#include <linux/limits.h> +#include <linux/magic.h> +#include <linux/unistd.h> + #include <bpf/bpf.h> #include <bpf/hashmap.h> #include <bpf/libbpf.h> /* libbpf_num_possible_cpus */ @@ -73,11 +76,73 @@ static bool is_bpffs(char *path) return (unsigned long)st_fs.f_type == BPF_FS_MAGIC; } +/* Probe whether kernel switched from memlock-based (RLIMIT_MEMLOCK) to + * memcg-based memory accounting for BPF maps and programs. This was done in + * commit 97306be45fbe ("Merge branch 'switch to memcg-based memory + * accounting'"), in Linux 5.11. + * + * Libbpf also offers to probe for memcg-based accounting vs rlimit, but does + * so by checking for the availability of a given BPF helper and this has + * failed on some kernels with backports in the past, see commit 6b4384ff1088 + * ("Revert "bpftool: Use libbpf 1.0 API mode instead of RLIMIT_MEMLOCK""). + * Instead, we can probe by lowering the process-based rlimit to 0, trying to + * load a BPF object, and resetting the rlimit. If the load succeeds then + * memcg-based accounting is supported. + * + * This would be too dangerous to do in the library, because multithreaded + * applications might attempt to load items while the rlimit is at 0. Given + * that bpftool is single-threaded, this is fine to do here. + */ +static bool known_to_need_rlimit(void) +{ + const size_t prog_load_attr_sz = offsetofend(union bpf_attr, attach_btf_obj_fd); + struct bpf_insn insns[] = { + BPF_EXIT_INSN(), + }; + struct rlimit rlim_init, rlim_cur_zero = {}; + size_t insn_cnt = ARRAY_SIZE(insns); + union bpf_attr attr; + int prog_fd, err; + + memset(&attr, 0, prog_load_attr_sz); + attr.prog_type = BPF_PROG_TYPE_SOCKET_FILTER; + attr.insns = ptr_to_u64(insns); + attr.insn_cnt = insn_cnt; + attr.license = ptr_to_u64("GPL"); + + if (getrlimit(RLIMIT_MEMLOCK, &rlim_init)) + return false; + + /* Drop the soft limit to zero. We maintain the hard limit to its + * current value, because lowering it would be a permanent operation + * for unprivileged users. + */ + rlim_cur_zero.rlim_max = rlim_init.rlim_max; + if (setrlimit(RLIMIT_MEMLOCK, &rlim_cur_zero)) + return false; + + /* Do not use bpf_prog_load() from libbpf here, because it calls + * bump_rlimit_memlock(), interfering with the current probe. + */ + prog_fd = syscall(__NR_bpf, BPF_PROG_LOAD, &attr, prog_load_attr_sz); + err = errno; + + /* reset soft rlimit to its initial value */ + setrlimit(RLIMIT_MEMLOCK, &rlim_init); + + if (prog_fd < 0) + return err == EPERM; + + close(prog_fd); + return false; +} + void set_max_rlimit(void) { struct rlimit rinf = { RLIM_INFINITY, RLIM_INFINITY }; - setrlimit(RLIMIT_MEMLOCK, &rinf); + if (known_to_need_rlimit()) + setrlimit(RLIMIT_MEMLOCK, &rinf); } static int diff --git a/tools/include/linux/kernel.h b/tools/include/linux/kernel.h index 4b0673bf52c2..5c90d65cc2d3 100644 --- a/tools/include/linux/kernel.h +++ b/tools/include/linux/kernel.h @@ -24,6 +24,11 @@ #define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER) #endif +#ifndef offsetofend +# define offsetofend(TYPE, FIELD) \ + (offsetof(TYPE, FIELD) + sizeof(((TYPE *)0)->FIELD)) +#endif + #ifndef container_of /** * container_of - cast a member of a structure out to the containing structure
Bpftool used to bump the memlock rlimit to make sure to be able to load BPF objects. After the kernel has switched to memcg-based memory accounting [0] in 5.11, bpftool has relied on libbpf to probe the system for memcg-based accounting support and for raising the rlimit if necessary [1]. But this was later reverted, because the probe would sometimes fail, resulting in bpftool not being able to load all required objects [2]. Here we add a more efficient probe, in bpftool itself. We first lower the rlimit to 0, then we attempt to load a BPF object (and finally reset the rlimit): if the load succeeds, then memcg-based memory accounting is supported. This approach was earlier proposed for the probe in libbpf itself [3], but given that the library may be used in multithreaded applications, the probe could have undesirable consequences if one thread attempts to lock kernel memory while memlock rlimit is at 0. Since bpftool is single-threaded and the rlimit is process-based, this is fine to do in bpftool itself. This probe was inspired by the similar one from the cilium/ebpf Go library [4]. [0] commit 97306be45fbe ("Merge branch 'switch to memcg-based memory accounting'") [1] commit a777e18f1bcd ("bpftool: Use libbpf 1.0 API mode instead of RLIMIT_MEMLOCK") [2] commit 6b4384ff1088 ("Revert "bpftool: Use libbpf 1.0 API mode instead of RLIMIT_MEMLOCK"") [3] https://lore.kernel.org/bpf/20220609143614.97837-1-quentin@isovalent.com/t/#u [4] https://github.com/cilium/ebpf/blob/v0.9.0/rlimit/rlimit.go#L39 Cc: Stanislav Fomichev <sdf@google.com> Cc: Yafang Shao <laoar.shao@gmail.com> Suggested-by: Daniel Borkmann <daniel@iogearbox.net> Signed-off-by: Quentin Monnet <quentin@isovalent.com> --- tools/bpf/bpftool/common.c | 71 ++++++++++++++++++++++++++++++++++-- tools/include/linux/kernel.h | 5 +++ 2 files changed, 73 insertions(+), 3 deletions(-)