Re: verifier: variable offset stack access question


Yonghong Song
 

On Wed, Dec 23, 2020 at 2:21 PM Andrei Matei <andreimatei1@...> wrote:

Hello Yonghong, all,

I'm curious about a verifier workaround that Yonghong provided two years ago, in this thread.
Brendan Gregg was asking about accessing stack buffers through a register with a variable offset, and Yonghong suggested a memset as a solution:
"
You can initialize the array with ' ' to workaround the issue:
struct data_t data;
uint64_t max = sizeof(data.argv);
const char *argp = NULL;
memset(&data, ' ', sizeof(data));
bpf_probe_read(&argp, sizeof(argp), (void *)&__argv[0]);
uint64_t len = bpf_probe_read_str(&data.argv, max, argp);
len &= 0xffffffff; // to avoid: "math between fp pointer and register errs"
bpf_trace_printk("len: %d\\n", len); // sanity check: len is indeed valid
"

My question is - how does the memset help? I sort of understand the trouble with variable stack access (different regions of the stack can hold memory of different types), and I've looked through the verifier's code but I've failed to get a clue.
I cannot remember details. Here, what "memset" did is to initialize
related bytes in stack to 0. I guess maybe at that point
bpf_probe_read_str requires an initialized memory?

Right now, bpf_probe_read_str does not require initialized memory, so
memset may not be necessary.


As far as actually trying the trick, I've had difficulty importing <string.h> in my bpf program. I'm not working in the context of BCC, so maybe that makes the difference. I've tried zero-ing out my buffer manually, and it didn't seem to change anything. I've had better success allocating my buffer using map memory rather than stack memory, but I'm still curious what a memset could do for me.
A lot of string.h functions are implemented as external functions in
glibc. This won't work for bpf programs as the bpf program is not
linked against glibc. The clang compiler will translate the above
memset to some stores if memset() size is not big enough. Better,
using clang __builtin_memset() so it won't have any relation with
glibc.


Thanks a lot!

- Andrei

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