Philipp Rudo <prudo@xxxxxxxxxx> writes:
> On Wed, 15 Dec 2021 11:18:34 +0100
> Sven Schnelle <svens@xxxxxxxxxxxxx> wrote:
>
>> slurp_file() cannot be used to read proc files, as they are returning
>> a size of zero in stat(). Add a function slurp_proc_file() which is
>> similar to slurp_file(), but doesn't require the size of the file to
>> be known.
>>
>> Signed-off-by: Sven Schnelle <svens@xxxxxxxxxxxxx>
>> ---
>> kexec/kexec.c | 32 ++++++++++++++++++++++++++++++++
>> 1 file changed, 32 insertions(+)
>>
>> diff --git a/kexec/kexec.c b/kexec/kexec.c
>> index f63b36b771eb..a1acba2adf2a 100644
>> --- a/kexec/kexec.c
>> +++ b/kexec/kexec.c
>> @@ -1106,6 +1106,38 @@ static void remove_parameter(char *line, const char *param_name)
>> }
>> }
>>
>> +static char *slurp_proc_file(const char *filename, size_t *len)
>> +{
>> + ssize_t ret, startpos = 0;
>> + unsigned int size = 64;
>> + char *buf = NULL, *tmp;
>> + int fd;
>> +
>> + fd = open(filename, O_RDONLY);
>> + if (fd == -1)
>> + return NULL;
>> +
>> + do {
>> + size *= 2;
>> + tmp = realloc(buf, size);
>> + if (!tmp) {
>> + free(buf);
>> + return NULL;
>> + }
>> + buf = tmp;
>> +
>> + ret = read(fd, buf + startpos, size - startpos);
>> + if (ret < 0) {
>> + free(buf);
>> + return NULL;
>> + }
>> + startpos += ret;
>> + *len = startpos;
>> + } while (ret == size);
>
> I don't think this will work as intended.
>
> 1) read returns the bytes read. So ret has a maximum value of
> size - startpos and thus can only be equal to size on the first pass
> when startpos = 0.
>
> 2) it's not an error when read reads less bytes than requested but can
> happen when, e.g. it get's interrupted.
>
> The simplest solution I see is to simply use 'while (ret)' Even when
> that means that there is an extra pass in the loop with an extra
> call to realloc.
>
> The cleaner solution probably would be to put the read into a second
> loop. I.e. the following should work (no tested)
>
> do {
> [...]
> do {
> ret = read(fd, buf + startpos, size - startpos);
> if (ret < 0) {
> free(buf);
> return NULL;
> }
> startpos += ret;
> while (ret && startpos != size);
>
> *len = startpos;
> } while (ret);
>
> Thanks
> Philipp
Sigh. True. I'll prepare a v3...
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