2022-10-18 14:55:13

by Xu Kuohai

Subject: [PATCH] libbpf: Avoid allocating reg_name with sscanf in parse_usdt_arg()

From: Xu Kuohai <[email protected]>

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 <[email protected]>
Signed-off-by: Xu Kuohai <[email protected]>
---
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, &reg_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, &reg_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, &reg_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, &reg_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

2022-10-18 18:52:52

by Stanislav Fomichev

Subject: Re: [PATCH] libbpf: Avoid allocating reg_name with sscanf in parse_usdt_arg()

On 10/18, Xu Kuohai wrote:
> From: Xu Kuohai <[email protected]>

> 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 <[email protected]>
> Signed-off-by: Xu Kuohai <[email protected]>

Addresses Andrii's suggestion from the following:

https://lore.kernel.org/bpf/[email protected]/

Acked-by: Stanislav Fomichev <[email protected]>

> ---
> 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,
> &reg_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, &reg_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,
> &reg_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, &reg_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

2022-10-21 22:00:48

by patchwork-bot+netdevbpf

Subject: Re: [PATCH] libbpf: Avoid allocating reg_name with sscanf in parse_usdt_arg()

Hello:

This patch was applied to bpf/bpf-next.git (master)
by Andrii Nakryiko <[email protected]>:

On Tue, 18 Oct 2022 10:55:38 -0400 you wrote:
> From: Xu Kuohai <[email protected]>
>
> 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.
>
> [...]

Here is the summary with links:
- libbpf: Avoid allocating reg_name with sscanf in parse_usdt_arg()
https://git.kernel.org/bpf/bpf-next/c/d9740535b857

You are awesome, thank you!
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