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Merge #19157

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19157: sys/shell_lock: do not call strlen, less jumpy r=benpicco a=kfessel

### Contribution description

the current `_safe_strcmp` depends on not being optimized and not being inlined (implicitly given by the -O0), this new one does less (would say not since even O3 had compile results that should not return early or show different runtimes for different secrets).

the runtime of strlen depend on the length of the string (password) therefor it is removed. `ifs` are very jumpy and depend on the contend of password, this avoids `ifs` sadly clang still compiles some boolean statements to if.  

with this PR the `__attribute__((optimize("O0")))` should be removable. 

### Testing procedure

see https://godbolt.org/z/x35b85cEx
and run the `<RIOT>/tests/shell_lock` 

### Issues/PRs references
#19155 made me aware of this function 

Co-authored-by: Karl Fessel <karl.fessel@ovgu.de>
This commit is contained in:
bors[bot] 2023-01-17 16:15:45 +00:00 committed by GitHub
commit 1c7300bb41
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1 changed files with 8 additions and 20 deletions
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28
sys/shell_lock/shell_lock.c
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@ -81,35 +81,23 @@ static bool __attribute__((optimize("O0"))) _safe_strcmp(const char* input, cons
{
bool the_same = true;
int input_len = strlen(input);
int pwd_len = strlen(pwd);
int input_index = 0;
int pwd_index = 0;
do {
if (input[input_index] != pwd[pwd_index]) {
the_same &= false;
}
else {
the_same &= true;
}
the_same &= input[input_index] == pwd[pwd_index];
/* keep indices at last index of respective string */
if (input_index < input_len) {
input_index++;
}
input_index += input[input_index] != '\0';
pwd_index += pwd[pwd_index] != '\0';
if (pwd_index < pwd_len) {
pwd_index++;
}
} while (input[input_index] != '\0' );
} while (input[input_index] != '\0');
/* ensure last char is the same */
the_same &= input[input_index] == pwd[pwd_index];
if (input_len != pwd_len) {
/* substring of the password doesn't count */
return false;
}
/* ensure last index is the same */
the_same &= input_index == pwd_index;
return the_same;
}