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Tue, 18 Oct 2022 11:32:47 -0700 (PDT) Date: Tue, 18 Oct 2022 11:32:46 -0700 In-Reply-To: <20221018145538.2046842-1-xukuohai@huaweicloud.com> Mime-Version: 1.0 References: <20221018145538.2046842-1-xukuohai@huaweicloud.com> Message-ID: Subject: Re: [PATCH] libbpf: Avoid allocating reg_name with sscanf in parse_usdt_arg() From: sdf@google.com To: Xu Kuohai Cc: bpf@vger.kernel.org, linux-kernel@vger.kernel.org, Alexei Starovoitov , Daniel Borkmann , John Fastabend , Andrii Nakryiko , Martin KaFai Lau , Song Liu , Yonghong Song , KP Singh , Hao Luo , Jiri Olsa Content-Type: text/plain; charset="UTF-8"; format=flowed; delsp=yes Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10/18, Xu Kuohai wrote: > From: Xu Kuohai > The reg_name in parse_usdt_arg() is used to hold register name, which > is short enough to be held in a 16-byte array, so we could define > reg_name as char reg_name[16] to avoid dynamically allocating reg_name > with sscanf. > Suggested-by: Andrii Nakryiko > Signed-off-by: Xu Kuohai Addresses Andrii's suggestion from the following: https://lore.kernel.org/bpf/86c88c01-22eb-b7f8-9c65-0faf97b4096b@huawei.com/ Acked-by: Stanislav Fomichev > --- > tools/lib/bpf/usdt.c | 16 ++++++---------- > 1 file changed, 6 insertions(+), 10 deletions(-) > diff --git a/tools/lib/bpf/usdt.c b/tools/lib/bpf/usdt.c > index 49f3c3b7f609..28fa1b2283de 100644 > --- a/tools/lib/bpf/usdt.c > +++ b/tools/lib/bpf/usdt.c > @@ -1225,26 +1225,24 @@ static int calc_pt_regs_off(const char *reg_name) > static int parse_usdt_arg(const char *arg_str, int arg_num, struct > usdt_arg_spec *arg) > { > - char *reg_name = NULL; > + char reg_name[16]; > int arg_sz, len, reg_off; > long off; > - if (sscanf(arg_str, " %d @ %ld ( %%%m[^)] ) %n", &arg_sz, &off, > ®_name, &len) == 3) { > + if (sscanf(arg_str, " %d @ %ld ( %%%15[^)] ) %n", &arg_sz, &off, > reg_name, &len) == 3) { > /* Memory dereference case, e.g., -4@-20(%rbp) */ > arg->arg_type = USDT_ARG_REG_DEREF; > arg->val_off = off; > reg_off = calc_pt_regs_off(reg_name); > - free(reg_name); > if (reg_off < 0) > return reg_off; > arg->reg_off = reg_off; > - } else if (sscanf(arg_str, " %d @ %%%ms %n", &arg_sz, ®_name, &len) > == 2) { > + } else if (sscanf(arg_str, " %d @ %%%15s %n", &arg_sz, reg_name, &len) > == 2) { > /* Register read case, e.g., -4@%eax */ > arg->arg_type = USDT_ARG_REG; > arg->val_off = 0; > reg_off = calc_pt_regs_off(reg_name); > - free(reg_name); > if (reg_off < 0) > return reg_off; > arg->reg_off = reg_off; > @@ -1456,16 +1454,15 @@ static int calc_pt_regs_off(const char *reg_name) > static int parse_usdt_arg(const char *arg_str, int arg_num, struct > usdt_arg_spec *arg) > { > - char *reg_name = NULL; > + char reg_name[16]; > int arg_sz, len, reg_off; > long off; > - if (sscanf(arg_str, " %d @ %ld ( %m[a-z0-9] ) %n", &arg_sz, &off, > ®_name, &len) == 3) { > + if (sscanf(arg_str, " %d @ %ld ( %15[a-z0-9] ) %n", &arg_sz, &off, > reg_name, &len) == 3) { > /* Memory dereference case, e.g., -8@-88(s0) */ > arg->arg_type = USDT_ARG_REG_DEREF; > arg->val_off = off; > reg_off = calc_pt_regs_off(reg_name); > - free(reg_name); > if (reg_off < 0) > return reg_off; > arg->reg_off = reg_off; > @@ -1474,12 +1471,11 @@ static int parse_usdt_arg(const char *arg_str, > int arg_num, struct usdt_arg_spec > arg->arg_type = USDT_ARG_CONST; > arg->val_off = off; > arg->reg_off = 0; > - } else if (sscanf(arg_str, " %d @ %m[a-z0-9] %n", &arg_sz, ®_name, > &len) == 2) { > + } else if (sscanf(arg_str, " %d @ %15[a-z0-9] %n", &arg_sz, reg_name, > &len) == 2) { > /* Register read case, e.g., -8@a1 */ > arg->arg_type = USDT_ARG_REG; > arg->val_off = 0; > reg_off = calc_pt_regs_off(reg_name); > - free(reg_name); > if (reg_off < 0) > return reg_off; > arg->reg_off = reg_off; > -- > 2.30.2